well, not having sex until 18 doesn't really matter, unless of course you're some kind of horny machine. other than that, the whole concept of making a bet with a witch actually surprised me there; what an interesting twist to the story. I'd say it hooked me from the start, plus, it ain't like Lust Geasse which had the same concept, but worse.
i just had to leave a comment here, that twist was ingenious.