I'm not too familiar with the adult-only services, but if you're interested in renting normal movies as well as adult ones, you might want to check out
www.greencine.com. They're like Netflix, but they have porn! They also do a cool thing for porn which is streaming video - you pay 2-3 dollars and can watch it right away (or after download) on your computer. They have a pretty good selection, although if you're only into porn you might want to stick with a dedicated service.
Bingle