The first impressions sometimes are the ones that stay. But not with Assassin's Creed.
My first impression was pure amazement. The city looked great and the missions were very compelling.
Until I got to the second city and realized that everything was just the same, with slightly different scenery: Go to the city, save some citizens, reach some high points to view the map, save more citizens, pickpocket, interrogate and assassinate. By the time your saving your 50th citizen it all just seems plain boring. The only thing that kept me going was the story, not saying that it doesn't have its issues though (the plot isn't presented to you in a very gripping way and you might even miss some key events that happen outside the animus).
And then there are those little annoyances in the cities: Madmen, Drunks and Beggars. I get their point, that is to be some kind of obstacle between you and your goal, but with such dull missions they just seem to be some cheap way to make the missions harder to achieve. Instead of creating unique missions on each city they just put them in your way.
Another thing I found really awful was the inability to swim, falling in the water is instant death. Now imagine a level that involves jumping between some boats to reach a bigger boat, and with Drunks in almost every boat and pier in your way! If you do as much as to get near them they'll push you into the water, to your death! And killing them also takes away some of your HP. Come on! Its not challenge, its lazy and cheap. It just seems very unrealistic: Altair is an Assassin for Christ's sake! Wouldn't he know how to swim?
But this game isn't a total disaster though. Platforming is very engaging and usually only involves your ability. Combat is also very well done.
I haven't had a chance to play Assassin's Creed II yet, but I've heard they've fixed the problems the first game had, mainly the tedious missions.
I just can't believe GameSpot gave it a 9! It is way too flawed to receive such score.