Sadly, after regular organic traffic, adwords traffic has to be one of the best available.
Neither Yahoo Search Marketing or Bing (msn) come close to matching it in quantity or quality.
I have been doing some mainstream PPC marketing for a while now and if the people in adult are surprised at cross sales and shady shit, you should see the kind of stuff people in mainstream are pulling. Endless rebills for BS pills like acai, putting a fake Oprah endorsement on the landing pages,etc.
Google is cracking down HARD on rebills for a few weeks now and the people in mainstream that were running that bullshit Acai thing are panicking. But most of them made a shitload of money.
I am still trying some other stuff on PPC that does well, but for some reason I keep coming back to adult
But the money is much easier in mainstream no doubt. The past two years were insane in mainstream.