I actually have a copy of Justine, which is the only de Sade I've read. I guess what struck me is that it was just a bit unrealistic in parts, to say the least. For example, a very large penis during anal sex with no lube? The abrasions would have hurt him almost as much as it did her.

Justine is like a submissive's journey through BDSM with many different doms, although she is always portrayed as being unwilling (and don't some players get off on pretending that the sub is unwilling?). It's a like a grand tour in a way, through every kink de Sade could think of, including even decapitation. Not surprisingly, many of the fetishes are still very much with us today, while others are still quite rare.

And of course, decapitation is quite illegal and not a viable part of the BDSM world. But just imagine if you got with a dom, a blind date perhaps, and then found out that he meant to really hurt you, maybe even kill you. It suddenly seems more relevant, doesn't it?

I haven't read O, but a friend once told me he thought O was partially inspired by the theme of Justine bouncing from dom(s) to dom(s).