I’ll try to keep this short and relatively spoiler-free. I liked it; I thought it was well-acted, well-cast (despite early doubts), and I wasn’t bothered by the changes to the original story. The movie stayed true to the spirit of the original work, and did a good job of communicating the major points of backstory that were cut for length. I’m even okay with the changes to the end, which probably means I’m not a TrueFan (regardless of my devotion to Alan Moore and my firm belief that he can in fact fly and shoot fire out of his eyes).
My nitpicks are relatively minor and have to do with Zack Snyder’s directorial style. The hyper-violence worked much better for 300 than it did for Watchmen, in my opinion, and at times detracted from the gravity/power of certain scenes. Related to that – the way the action scenes were shot made all the characters look super-powered, when the true strength of the story rests on the fact that they are only human. All in all though, thoroughly enjoyable, highly recommended, and it sure looked purdy up on the big IMAX screen.