It is, as Dom says, a very clever, subtle novel, with a number of great one-liners and paragraphs. The characters are well fleshed out and have strong individual voices, particularly the protagonists Valmont and de Merteuil. It does have the characteristics that you might expect from a 17th century novel, even a Libertine one - i.e., it's wordy, and even though the protagonists are also the bad guys, the conclusion is rather moralizing where a contemporary author would likely have allowed vice to triumph over the stifling requirements of orthodox society.

As for graphic, no, can't say that it is; the letters tend to be clear enough as to leave not much room for doubt about what is going on, sometimes with rather clever euphemisms, but you won't find a single graphic word in them (although in one passage Valmont congratulates himself on having taught his latest virgin conquest the sexual vocabulary in such explicit terms that her upcoming wedding night to another man ought to become an interesting affair).

Honestly, I wouldn't recommend this to most people, even though I enjoyed it myself - at the end of the day it's 175 letters written in a late 18th century style, of which only a minority is piquant, funny or otherwise particularly noteworthy (letter 105 is the most impressive one of the bunch - probably the most vicious, manipulative and generally evil letter I've ever read). So it does require some patience, and as I said the ending is disappointing by modern standards. You'll appreciate it I'm sure, as long as you don't expect this to be what it's not.