I have actually found this game really enjoyable and I would thoroughly recommend it for it's different feel. It should be noted that it is a single player title with no Xbox Live or multiplayer integration at all, but this is no bad thing....at least it means all efforts have been channelled into making the game good.
Ubi Soft would obviously claim that Assassin's Creed has been built "from the ground up", but it's roots are clearly in the Tom Clancy's titles, especially Splinter Cell. This means you get decent physics, stunning lighting and quality textures on a richly 3D map, plus a solid, well thought-out story.
You play as Desmond Myles, descendent of Altair (al-tie-ear), who has inherited the memory of his ancestors through his DNA. These memories are sought out by a scientific reserach group who are after a lost treasure from ancient times. It's your role to access and play out these memories of ancient times, and in doing so you spend most of your time carrying out the tasks of Altair in the 'simulated' ancient realm.
This all happens in the time of King Richard, between the cities of Damascus (Turkey), Jerusalem, and Acre (England I think), with an expansive rural 'kingdom' connecting the cities, which you cross on horseback!
The combat is all about swords and knives and archery.....no grenades and rocket launchers here....so fights with city guards and the like tend to be pretty intense. The games' big feature is free running; you can scale pretty much anything and leap from roof top to roof top in an effort to disappear.
This is where you can see the re-use of Splinter Cell code....it's all about being low or high profile. If you are discreet you can pass through the cities unnoticed, blending with the crowds and not attracting the attention of any roof guards. Climbing ladders gets you to the rooftops where you can move around more freely, but as you would expect, it gets harder as the game progresses to not attract unwanted attention.
So like Splinter Cell where you use light and shade to control how visible you are, in Assassin's Creed you use discreet and non-discreet behaviour in the same way.
I've found this particularly enjoyable in Assassin's Creed, as you can change the pace of the game in an instant, whenever you like, and it makes the in-between times less tedious as you don't have to creep around everywhere.
There's plenty of side missions and objectives that are well worth doing, and a good haul of gamerscore points available, and also as you progress through the game, you unlock new weapons and abilities. Without this the game would probably get too repetative too quickly, but with this built-in control over your progress it actually helps you develop your skill at controlling Altair's actions.
As you spend more and more time playing, you will find that you can move around the environment much, much faster, and it's at this point where I've been disappointed at how regularly the game/disc has to catch up. It's only perhaps half a second that it hangs for, but it still messes with the flow, and I think it's a symptom of Ubisoft games, where they rush to meet a deadline and release a game with a few unfortunately obvious bugs. It's the same with the Rainbow Six Vegas and Ghost Recon AW stuff.
My only other complaint, as others have mentioned, is that they only recorded one set of dialogue for people that you re-encounter, so for example, when you save a citizen they'll say the same thing the last citizen said in a different city. For the sake of a couple of minutes of extra audio, it would have made a big difference.
Despite these negative points, I still give the game my full recommendation as it's a really fresh approach to 3rd person role-playing action adventure stuff, and the free running is really cool and well done.
The concept lends itself to a franchise, and it'd be great to see a sequel set somewhere different, like feudal Japan.