First, this is a really fun movie. It's not too horribly gross, (except maybe for the scene with the tongue), and it's not very creepy, or too scary, It's also not at all a comedy, though it is a bit tongue-in-cheek (in more ways than one). It's just, well... Fun!
If you've ever read any books about witchcraft, or seen old woodcuts of people's visions of witchcraft during the 16th century, you'll recognize the themes and images in this film. You'll get the witch hunter holding onto the flying warlock with a rope, nails being driven into footprints, the whole bit... And you really should look up the traditional main ingredient of the infamous flying ointment before watching.
This film works from the premise that everything in the old witchcraft books is literally true, and it plays out in a very entertaining, visually distinctive way. It was well-researched enough to really make the premise work, without being so slavish as to detract from the plot. The warlock is every bit as evil as the old books would have it, and is most definitely in league with The Devil Himself.
And I've got to say it: Julian Sands is just plain COOL.