Hellboy may be the first movie based on a comic book that is both faithful to its original source material and manages to be a pretty good movie. Usually you can get one or another -- something slavishly precise to the comic but dreadful in movie form, or a reasonably decent film (like X-Men or Spider-Man) that needs to take some liberties with the comic to work well as a movie.
While Hellboy isn't exactly identical to the comic, it's damned close. The origin sequence, taking up the first 15 minutes or so, is not only plotted like the original, it looks like the original. The character designs match quite precisely; the scenery could only be designed by Mike Mignola. (The comic's creator/writer/artist, who was Executive Producer for the movie, which presumably explains why it looks so right.)
And it is a good film: not high art, perhaps, but with a very nice mix of big action sequences and quiet human ones, including one of the shyest and funniest romantic bits I've seen in a while. (Nothing is quite so delicious as watching a seven-foot-tall red creature acting like Hugh Grant.) The characters are true to the original, but actually better in many cases: Abe Sapien's gentle alienation and Liz Sherman's fingertip grip on reality are both sharply and richly drawn. The bad guys are thoroughly over-the-top eeeeevil, but that too is true to the comic -- I mean, we're dealing with occult Nazis who are trying to summon Lovecraftian Elder Gods here. (And that's just in the first few minutes!)
This is very much a monster movie -- if you're squicked by tentacles, seek something else. But it's more about the action and the quiet humor rather than gore; indeed, there isn't a lot of actual blood in the film.
As ladysprite remarked, this is really a summer film come early -- an excellent chance to sit back with some popcorn and enjoy the ride. You'll like it, if you like this kind of thing. I was dreading the usual savaging of a comic that I respect a lot; instead, I find that just for once, the comic translates well to the big screen, and the studio had enough faith to let it do so. Thumbs up.
Trailer reviews: Winner -- Spider-Man 2, whose new trailer looks even better than the previous one did. Loser -- The Punisher, which may well be fairly faithful to the comic, but I completely can not find it in me to care. (Yeah, movies based on comics are really very, very in right now...)