I read the Watchmen graphic novel back in the late 80s and it made a big impression on me. However, when I heard that the film adaptation was coming out, I made a point of NOT rereading it again beforehand. I wanted to give the film a chance to stand on its own before I started making comparisons with the source material. I dl'd the Motion Comic so I can revisit the original tonight, but wanted to get my thoughts on paper first.
Anytime you have to adapt a richly layered and dense text to the big screen there are going to have to be compromises. If you try and put EVERYTHING on the screen, you are likely to end up with 5-8 hours of unwatchable mush. But if you leave too much out you risk alienating your core audience. The filmmaker has to walk a very delicate line by streamlining the action and exposition while still keeping true to the main themes of the original. I've seen it done well (LOTR) and poorly (Dune).
This one was done very well.
I have to agree with
cleolinda that part of the reason the critical reception is so sharply divided is the advertising. The ads HAD to contain some of the iconic images from the graphic novel in order to convince the fans that he was remaining true to the source material, but for those unfamiliar with the source it made it look like an Action Movie - Which it emphatically is NOT. It's a morality play with explosions.
( (I'm gonna get spoilery here) )