it's actually a good reference for studying human anatomy; when it comes to various postitions, actions and postures.

great help if you don't know how to anatomize breasts, hips, torso and legs...
oh yeah, almost forgot to mention, facial expression, pupil dilations, muscle contractions...

if you look past the sex, sordid storyline and language, hentai's pretty helpfull when it comes to improving your drawing.