What worked best for me was the material that showed Hellboy, portrayed exquisitely by Ron Perlman, as just a regular guy cum sassy detective. Even though he's half demon, a large part of the comics, at least--and this is hinted at in the film, particularly in the climax--is a continual nature versus nurture "debate". He was raised by humans who were as normal as they could be, being government agents in a bureau dedicated to the paranormal. So he has a large number of human-like quirks, including a love of old music, beer, cats, pancakes, chili, and so on. He's also a cigar-smoking, smart-assed detective. Hellboy is at its best when it focuses on these characteristics.
But everything else works well, too. Hellboy has a monster-like counterpart, Abe Sapien, and a "freak" love interest, Liz Sherman, who are almost as fascinating as he is. The villain and neutral creatures (such as the "half-creature" with a speaking role towards the end) are just as captivating. There are also other characters providing enjoyable comic relief, most notably Tom Manning (Jeffrey Tambor). His adopted father, Professor Trevor "Broom" Bruttenholm is intriguing. And newly recruited "caretaker" John Myers shows promise, even if we do not get to spend much time with him here. Like many films of this type, I'd love to see all of these characters further explored in prequels, sequels and spinoffs. That's a good sign, because it shows that del Toro has successfully conveyed a world with "deep" characters who have extensive histories.
Hellboy is captivating, suspenseful and humorous. It is well worth watching for anyone with a taste for fantasy. And you know I'll be at the theaters soon to see Hellboy II, which opens this weekend!