I saw it last night, and really liked it a lot. I saw it with a friend who had never read the comic, and I think he was just lost (and I think he expected a Fantastic Four or X-Men type of movie) though.
I could kind of tell that there was not only a lot of the comic story left out, but that there are things that were filmed that were left out. I expect that the seven and a half hour director's cut release will be a little more congruent. For the most part, though, I think that capturing as much of the book as they were able to do was quite an achievement.
I also have to say, I thought the modified ending worked well in the movie.
The only complaints I have are that Matthew Goode sounded like the goofy priest from The Princess Bride at times, and some of the old people make-up (e.g., Sally Jupiter, Nixon) looked kind of bad.
The Dr. Manhattan effects were great inasmuch as I never really thought of him as an effect.
Rorschach and Comedian could not have been cast any better, and I lost count of how many ways Malin Ackerman is hot.
I've already got tickets to see it later today in IMAX.