I've edited Mondain's impressions for everyone else.
This game is pretty weird. But it's got tons of innovative elements. It pretty much all works like you've been shown in the demonstration videos. I won't comment much yet because this seems like a longwinded game with a very slow pacing, but so far it plays like Grand Theft Auto set in Jerusalem in the age of crusades, with platforming elements from Ubisoft's Prince of Persia.
It will probably begin to show its magic later on, but so far it didn't. For instance, it doesn't feel epic, I hope that this will change. And, the pacing is very slow and dull, and the in-game cutscenes largely uninteresting.
Don't believe the bullshots too, Assassin's Creed graphics are pretty rough around the edges and devoid of detail in most locations. Of course, this is the price to pay for such a huge free-roaming universe with tons of characters on screen. On the other hand, the scope of the title is absolutely unbelievable, I don't think that there's anything that was ever made before that even approaches it in game form. You'll stop and look around you to see the city's detail and the amount of stuff in the background, and you'll think that this is truly a marvel.
The game's framerate is overally pretty slow, but relatively stable. There is some screen tearing but thankfully it's pretty infrequent. It's very far removed from the disaster that we feared and doesn't affect the experience.
Everything is shown via Altair's perspective as if he was a real person kinda like in Half-Life, only this time from a third person perspective, and without a silent hero. That does make the pacing pretty dull, but it makes the game very gripping, you'll have a very hard time to put the controller down. Suspense and immersion seem to be very important. For that reason I think it'd be better to pick the PS3 version of the game, because the game's aural portion is extremely quiet nearly all the time and the 360's DVD-ROM whining tends to overshadow the dialogue and spoils that element of tension. There's no option at all to put subtitles on, sadly.
Mina on OAs, who is usually a huge Sony antagonist, has played the PS3 version along with the 360 version, and claimed it's a very good port. Also, that while the graphical performance is a bit more sluggish, it's very very minor, overall both games would look the exact same and one would really have to be on the hunt for technical flaws to let this affect the experience.
Lastly, many people feared the sci-fi elements of the game. I won't comment on them because that would somewhat spoil the surprise, but I do agree that they're completely unnecessary, and that Assassin's Creed would have been better without them.
Now this game is getting REALLY really good. As some reviews pointed out, you're asked to do repetitive tasks, but it doesn't really feel tedious. If you hate gangsta sandbox games I don't think that you're going to be turned off at all by this. This game is EXTREMELY addictive, don't boot it up when you only have time for some short 20 minutes-long gaming session; be ready to have a few free hours in front of you. It really plays like GTA combined with Prince of Persia, a strangely entertaining combination.
A few hours in you enter the first big city (Damasca?) and then the game opens up exponentially. It's still rough around the edges, but the graphics also get much better in the city. The amount of stuff going on in the background is incredible! Your jaw will drop when you will first see that town on the horizon, it's going to be nearly as impressive as your first encounter with the main Hyrule overworld in Ocarina of Time. As you always have the same perspective and move relatively slowly (although Altair never gets tired from running), it really makes you feel like this is really you on screen, walking in unknown towns, forced to accomplish daring stunts with your wits and your feeble means.
The controls are all excellent, I like the clever use of the L trigger (to target people), and the R trigger (to run and jump, mostly). Everything is very precise. Horseback riding is fun and everything responds very well. You have plenty of options to make yourself discrete when moving, or to hide yourself away when guards are near, and they come in very handy. You can really jump atop any building if you want to and it's always satisfying and spectacular to look at.
The crowd interactions are a joy to behold. There's so much attention to detail! You'll spend hours just staring at the little touches that make these towns feel alive. Even if it quickly becomes a string of pretty repetitive fetch quests and simple investigations to accomplish, it still remains truly great. Stealth kills are fun to accomplish, there are so many different strategies that you can use to accomplish your goal that it's mesmerizing. It would probably be nearly just as entertaining to replay through the single player quest twice.
An instant classic that deserves to be remembered for years to come, my favorite game of the year so far after Virtua Fighter 5 and Minna no Golf 5.
However, those third person in-engine cutscenes that all happen from the same perspective are really awkward, and they're everywhere in the game. It's seriously hard to pay attention to what's going on; surely it's somewhat a voluntary design choice (to force you to pay attention to your surroundings), but it could've been better. It works well when it's done at the first person in Oblivion, Half-Life or Call of Duty, but here it's just distracting and confusing.
And the sci-fi elements still feel incredibly out of place, they only serve to distract the player from the beautiful medieval atmosphere.
