Not having read the Mike Mignola comic book I can't say whether the movie is true to its source. If it's not though it would have to be a noble effort.
If Buffy had been a seven foot, cigar smoking demon from hell I'd imagine BTVS would turn out something like Hellboy. The franchises share a similar sensibility - a sardonic mix of cheesy horror, humour, action and relationships. Visually the movie had a nice texture to it as well.
At the end of it though I felt the film lurched along somewhat lethargically. I never felt for the main characters as much as I was meant to. Still, I'd be interested in giving the comic a go now.
edit: Why did John Hurt look like Mister Miyagi?