Spoiler free review, because it's nice to push "new game" not knowing what you're about to discover.
Assassin's creed makes a good impression right from the beginning; not many games immerse the player in an original setting, with interesting locations, complex gameplay and an deep storyline. From a gameplay point of view, the game expands the Prince of Persia recipe, refines it, in a free roaming universe, placed sometimes during the Crusades.

Speaking of this game, let's ponder first the following motto : "It doesn't matter WHAT you're doing, it matters HOW you're doing it". If this sounds ok to you, there's a big chance you'll be quite impressed by this game. If not, there's an equal chance to abandon it after a few hours, pissed off and frustrated by the wasted potential of the game. And that's because it's obvious AC could have been an instant classic, a game emulated or copied by the competition, but in it's current form, it's only practical purpose is serving as a launch pad for the sequel. It's a good game, but not an extraordinary game, close, but not close enough.

Assassin's Creed major problem is repetitivity . Everything which doesn't add up to the universe, animations, combat system or the main quest is horribly repetitive. It's quite obvious, unfortunately, regardless of how optimistic you're looking at the problem. There are 4 types of secondary missions: interrogation, eavesdropping, pickpocket and save the citizen. All of them repeat endlessly. Not even the NPCs speech is different, it makes your head hurt. You're running around the city, and you're listening the same town cryer message over, and over, and over again, it's making your ears bleed. And the icing on the cake is some central market in Jerusalem, where there are 2 of these guys, place a few meters apart, saying the exact same thing, with a few seconds delay: medieval cacophony.
Everything random in this game repeats itself.

The above paragraph represents the Achilles heel of this game. It's the "game breaker", the element which will make a lot of people just give up. With a little bit of patience however, those of you who will close their eyes to these inconveniences, and will discover the story, and the universe, and all the different things you can do will not be disappointed. As follows:

I don't know by what extent the producers' rants about the authenticity of the universe are true, but I know this: the moment you enter the game, the impression of "I'm here, it looks real" is quite obvious. Starting with the architecture, and the NPCs clothes, and the interior design of the buildings. It seems authentic, and it goes a long way immersing the player in the universe. Another great thing is the way the game's 4 major cities (Jerusalem, Damasc, Acre, Masyaf) are populated. We all know the publicity around other games trying to recreate a "living, breathing world", but Assassin's Creed pushes the concept to a new level. Every single city is full of non playing characters walking around, doing their daily routine; there is no complicated system of "now I sleep, now I eat", like in Oblivion, but one thing is clear: it's way better than Bethesda's game, because of the sheer number of them, and the way they just seem...natural, doing their thing. There's literally tons of NPCs roaming the streets in Assassin's Creed; there's so many of them you literally have to push your way through them, trying to get out of the crowds. They scream, shout and run around, they drop what they're carrying if pushed, and if the situation becomes dangerous (for them), they call the guards. Sure, after a few hours of gameplay you're already sick and tired of hearing the same tired old line used when an NPC calls for help, but at least from a visual standpoint, everything looks and feels great.
And, when the city's crowds start to get on your nerves,(there are characters especially designed to piss you off, like the beggars) you can always go the way of the roofs. And once you reach the roofs, you realize there is no other game which manages to even get close to what Ubisoft Montreal has managed to achieve with the game's free running system. Altair, the main character, runs without interruption on any surface, "switches" from different types of animations without any hitches, without seeing where the different frames change, without those little "dead" moments between animation types, with such fluidity and grace it leaves you floored in front of your TV. You move your character using the left stick, you hold Right Trigger, and push A. By doing that, Altair will correctly detect the surface he's running on, every single time, and will run, jump or climb according to the situation. You don't have to do anything; the game knows if there is a wall in front of the character, a ledge, a big drop from the roof, or anything else. And Altair will continue running, regardless of the obstacle, in such a natural and great way it makes you wish this game had a "replay" feature. It's impossible not being impressed at least a little when you see Altair running through the city. It's impossible not to appreciate the amazing freedom you have, the mind blowing animations, and the fact that there's virtually no surface you can't climb. And if you really can't, it just means you would have fallen out of the map if you did. The sheer number of different animations is mind blowing; after over 20 hours of play, right at the end of the game, in the final battle, before the credits, I saw a new combat move. There are THAT many.

Assassin's Creed encourages you to explore with the amazing freedom you have, to do anything, be it on the city streets, or up on the roofs. Besides, the only way to "discover" the map, and it's points of interest is by climbing the tallest building in the area, where a "discover" option appears, and you scout around the city before you with an impressive fly-by camera. You do this thing a lot in Assassin's Creed, but in never gets boring, because the game is so well put together from this point of view you just sit there watching, in awe, how Altair climbs what seemed to be an impossible ascent. The main character grabs on the smallest pieces of rock, the smallest ledges or any other element of that wall which offers grip it in such a natural way you just want to do it again. And I never ever saw him climb something "the wrong way". If there is no surface which might offer a bit of grip, it means that surface can't be climbed. The synchronisation between Altair's animations and the actual landscape is almost flawless. Impressive.
The combat system is equally fluid, and impressive. The entire system is based on counter attacks and precise timing of your hits. Classical attacks can be performed, as well as dodging, and grabbing/throwing your opponents. The heart of the system however is the defensive stance Altair has in each battle, waiting for his opponent to strike, dodging the blow and then executing him with an extremely brutal a precise attack. The actions which can be performed during a fight (and during the entire game, for that matter) are contextual, meaning the buttons' mapping is changed depending on what you do on screen. It sounds complicated, but in fact it really isn't. A fight is won (or lost) not because of the poor interface, but the skill the player has to precisely time his attacks, and dodge his opponents. And, speaking of which, I really hope this interface will be properly ported to the PC, when it will hit stores, next year.
Altair's arsenal isn't especially complex. He only has a sword, two types of knives (regular one, and the throwing variety), and the assassination spike, hidden under his sleave. If you feel particularly skillful during a fight, the spike can be used for counter attacks, which is quite difficult, as a missed blow leaves you open to the opponent's sword, not having a proper weapon to block his attack with.
Speaking of the assassination spike, the game is called Assassin's Creed for a reason. You have 9 distinct targets who must be assassinated throughout the land, for reasons I'm not going to go into, because I'll have to ruin the story. Each target has it's story, shown by usually kick-ass cutscenes before the actual assassination, and different ways to be reached, and killed.
All the secondary missions I was talking about earlier server to gather information about your target. Not all of them must be completed in order to "unlock" the assassination, however. You only need the basics: where the target is, when it will show up, and preferably how many people are defending it. For the perfectionist, if all the secondary missions are accomplished, there are achievements crafted exactly for this purpose. The assassinations start with unguarded merchants roaming through the city, and then move on to important military figures, heavily guarded in fortresses and such, towards the later part of the game. After the cutscene, which can be partially controlled by the player, you're free to assassinate the target as you see fit, and then the chase starts. The whole city is alerted to your presence, and you have to dodge (or fight) your pursuers until you find a safe spot to hide, and lose them. The chase scenes are great, but entirely skip-able if you don't feel like running through half the city to escape; kill a few guards, jump on a roof, and hide. It's all up to you.
The main story of the game is very interesting, built around these 9 targets, and it's gradually shown, without telling you to much, but also trying to suggest that all is not as it seems. And that works really well.

As mentioned earlier, the game universe is great, and to that end the graphics play a significant part. Not necessarily from a technical point of view, which isn't exactly impressive compared to other 360 (or PC) games, but form an artistic point of view. The art design is excellent, and it really helps. It wouldn't have hurt to have more NPCs constructed with a bit more detail, and also the area between the cities (because you have that, too) could have been better if textured with a little more detail, but overall the graphics are excellent. Sure, the far off textures (like the ones over the city walls, where you can't get) look like a turd, and the game has quite a number of small (but noticeable) graphical glitches, but it's not enough to put you off. Of course, having graphical errors (small as they are) in a console game isn't exactly nice, with the unified configuration and all, but hey, not everything's perfect.
The sound is also good, fortunately enough. The music consists mainly out of atmospheric tunes, and a few heart pumping chase sequences here and there. It serves it's purpose. The same can be said for the general sound FX category, which really help in making you feel be in a leaving, breathing place.
The mediocre part is the voice acting. Overall, it's not bad, but the thing that ruins it is the repetitivity. The problem is bigger than you might think: on one hand, the game immerses you in this believable world, and then it bangs your head against the speakers with the NPCs saying the same thing over and over, with Altair's American accent, and the way english is used in certain places where, surely, the original language of the area might have been better. I'm not saying this because I woke up on the wrong side of the bed: the game does have "localized" speech from time to time, such as the Templar knights (really hard to beat) which speak in French, or German, depending on their country of origin, or the city guards who also seem to speak in their native tongue. But this isn't consistent. You sometimes hear it as it's supposed to be, and sometimes you don't.
It's quite possible I'm overreacting, but this is a Triple A title after all, a little more attention to details such as these wouldn't have hurt a bit.

And now, before the end, what sucks, and what doesn't:
The good part:
Everything which helped build the game's sandbox is exceptional. The animation system redefines what you might have thought to be possible, on any platform. The combat system is great. The story is interesting and keeps you guessing right until the end, and the great "secret" which most gaming sites have been quite keen in ruining when writing about the game is worth it, bringing additional depth and "cool factor" to the game.
For these reasons alone, Assassins' Creed is worth your money.
The bad part:
Everything which doesn't add up to the main quest is repetitive and boring. The game has a truckload of different animations, but only a few decent sound samples for the characters in the game world. The secondary quests are built using ctrl-c and ctrl-v. The artificial ways of prolonging the game's length (different flags which must be collected, much like the hidden packages in GTA) are rather crappy, and add up to a full time job if someone might want to collect them all.
There are the relatively stupid game design situations which show up from time to time. You mostly ignore them, but sometimes it's a bit stupid to kill someone in the middle of the public square but not being noticed by anyone (although you're standing right next to the dead guy) just because you quickly pressed A (blend). Or, you're climbing on a guarded tower, the guards are looking at you, the "warning" HUD element flashes, and...nothing happens. Or the soldiers who politely wait their turn in attacking you just because the game isn't close to the end, and someone decided they shouldn't be too hard on the player.

So. Is it worth it ?
It most definitely is. You won't get the perfect game, but you WILL get a very good one, original in it's setting, and quite exceptional at times. And that's something you don't see very often these days.