Just started playing it, just got to Damascus. Very beautiful graphics, the game play seems pretty solid, but I still need to get all of the controls figured out.
I think Gabe addressed that POV pretty well in his post here: http://www.penny-arcade.com/2007/11/14/ (second post on the page, "AC") He says it better than I can.
I found that I didn't like the flag missions, so I stopped doing those and started doing things I found more enjoyable (pickpocketing, saving citizens, and finding really awesome arial views of the game world) I really enjoyed it. I played the game through once to the second to last mission and my PS3 broke... then played it all the way through again after warranty repair and beat it. Awesome game.
I'm finding it's just a tad repetitive... and easy The guards are a joke unless it's 1 v 10. I just got the throwing knives though, so I can finally do a few small ranged attacks.
The graphics are absolutely stunning though. Not quite Crysis' level, but hell, nothing will be for another year or two at least.