The only adult I ever did was organic, and I was #3 for a pretty high volume adult search term, and even at that I was only making $600/mo. The problem was that I was writing everything by hand and the site didn't really attract any natural links. I was able to get to that spot in the SERPs in about 9 months, which is good, but I don't think I'd be able to get to #3 for "boobs" or "teens" or something in that time frame.
I looked into scaling my operation, but I honestly don't know if I want to spend THAT much time looking at friggin pr0n. A shame too, because I figured out how to scale it and could have turned that $600/mo into $6000/mo relatively easily.
If I could have done it with one site in one niche, I would have. I'm just not the kind of guy who can sit there looking at all kinds of niche and sub-niche porn all day long.