I found the Marquis' work rather dry in places, but he had an exceptional way with words (and twisting expectations), so you shouldn't be too put off by what you hear and read about his books. Kinda reminds me of the way in which Alasteir Crowley's stuff is always regarded as being hardcore devilry when it is, in a lot of ways, a barbed and witty commentary on religion and personal expression.
I'm finding it hard to describe these guys, but I really, really believe that their work isn't confined to students and collectors of esoterica. There might not be as much in their work that comes close to the general perception of them (and you can forget the film adaps like Justine, because those were soft-porn first and foremost). There is some degree of 'nasty' in de Sade, but there is just as much laugh-out-loud comedy in his work as well.
I may be in the minority on this...