HELLBOY is probably everything Robert Bloch (from the intro to Seed of Destruction) and Alan Moore (from the intro to Wake the Devil) say it is but the bottom line is that it's just great fun; a great visual and mythical experience. It clears itself of any pretentiousness right of the bat: it's about a monster (more specifically, a demon) who goes around and beats up or gets beaten up by other monsters. Mike Mignola himself said he "wanted to draw monsters" and HELLBOY assures that he'll be able to do so, whether drawing the good guy or the bad guys. But while Hellboy may not be the most cerebral comic book on the rack (not to imply that there are that many to begin with), it draws it's inspiration from mythology and folklore, so it's definitely food for the brain. And it may just inspire you to go and read the source material.
Something must also said for the artistic effort in the SoD series. Not to say that subsequent HELLBOY stories drawn by Mignola look bad (far from it), but in Seed Of Destruction, one can clearly see the labor of love involved. The combination of line work and heavy blacks seem to convey a stronger sense of depth than usual. Speaking of heavy blacks, Mignola has admitted that such a technique began as a way to cover up what he "didn't know how to draw", but he certainly knows how to turn this admitted weakness into strength. Mignola also skimps on backgrounds, often just leaving them out for panels at a time, but when he does render backgrounds, he knows just how to set the mood, whether the background in question is a derelict church, a moonlit forest or a graveyard. So effective are Mignola's backgrounds when they are seen, that the reader barely misses them when they're absent.
Anyway, enough gushing praise. I end with a recommendation: anyone who's a fan of comics, folklore or mythology should give HELLBOY: SEED OF DESTRUCTION (or anything HELLBOY, for that matter) a look.