Depends what you mean: as a writer, as an erotic visionary or as a philosopher? The French regard him as a philosopher, but the rest of the world has trouble taking that seriously. Personally I find all those abstract dialogues about free will and personal liberty tough going, but then that's the French for you....
Well i sugest Justine to start with, The 120 Days Of Sodom they say is much more tough, since it seems to present endless kinds of torture, Justine while it has some of that, is more balanced, while presenting this and the philosphy. Don't know about the letters, i still haven't read them. But yes i think he was a very funny man, his books shouldn't be taken all serious, some of the dialogues on Justine were pretty funny, in a very disturbing way that is.I only came to know more about his life and ideas via a couple of biographies I read after some novels so I would appreciate the novels and the philosphy more now. I should revisit the novels now. Johannes is right, the letters are amazing. (The biographies include some.) He was a very difficult man. He really was operating on the limits of society and sanity if he was absolutely serious about all his ideas. I think he is often dismissed out of hand because of the sexual aspect. That said, if the sexual aspect did not permeate his writing and thought I think most of his books would be out of print...
I've wondered also about connections to Bukowski. I can't find any references indicating Bukowski ever read him
[...] morality,religion and laws were just invented to stop men act upon his most primordial instincts, his "natural" state. I think he was really one of the first to develope this thought about human nature. [...]