It's pretty good. Guillermo del Toro's take on Hellboy is as much del Toro as it is Mignola, I think--dank sewers, amber lighting, a kind of exalted lucidity in the action/fight sequences. He's easily a better action filmmaker than Robert Rodriguez--Del Toro gets the same kind of clarity in his combat footage that John Woo gets, without aping Woo the way Rodriguez does.
According to an interview, del Toro added the character of the rookie FBI agent, the love triangle, the paternal relationship between HB and his discoverer, and the spiky relations between HB and the FBI director. In other words, almost everything that gives the movie emotional heft (I'm sure the comic book has it too, but then it has as many pages as it needs to develop that heft, while the movie only has 132 minutes). The tone, though, the basic concept and the basic look I assume is Mignola's. And I assume Ron Perlman's wonderfully droll performance takes its cue from Mignola's eponymous character.
Easily the best comic movie I've seen since Burton's Batman flicks.