Now, before I go further, it's probably best that you watch the trailer for the movie. Watched it? Okay, sweet.
The movie manages to capture a lot of what makes the book so special. It's got that same feeling of "non-reality" from the comics, which is impressive. The cast, for the most part, is spot-on with the book, to the point where you can merely glance at any of the actors and say "ah, they're that character."
There's a few oddities, though. First off, the plot is so condensed (squeezing six thick volumes of the comic into two hours of film) that the "high points" of the comic become the standard tone for the book. Whereas the book was maybe 70% "normal" and 30% "crazy", the movie is about 20% normal and 80% crazy. It's not a huge problem, but it does get to a point where the rapid-fire insanity is almost tiring.
Another problem with the film is that the sixth and final volume of the comic was not yet completed at the time of filming, and thus the film's ending is slightly different. Both versions end up the same in the end, but certain very important details are left out.
Scott in the comics is a lazy, geeky, sometimes-energetic, slightly-dorky-yet-lovable guy. In the film, he's a dopey, selfish jerk. Cera's portrayal makes Scott look like he's just an arrogant, insecure jackass. The dialogue is exactly the same as in the book, but somehow Cera manages to make it seem terrible.
Now, aside from the (negative) comparisons to the original book, the movie is highly entertaining. The visual style alone makes the movie worth watching, but the dialogue and overall story make the whole thing border on the edge of brilliance. If the last act had retained the emotional closure and character depth that it should have (and Cera hadn't been cast), the film would be amazing. As it stands now, it's just "really fun, and pretty good."