I haven't read poisontaster's 'verse, but frankly the idea of a slave not purchased (only) for sex is more intriguing to me. probably because it's less of a trope. not to say that tropes are bad ;) but it's a rare sight and I definitely like that.

you're gonna love it! Damien and Leïto are both amazing as characters, also very slashy. the movie has loads of Parkour and martial arts (Cyril did the choreography for the stunts himself), and there are definitely fewer WTF? moments than in your average action movie.
while you're at it, you can buy its sequel Banlieue 13 Ultimatum as well :)