I have limited experience with purchased traffic, but I'm more than willing to talk about it.

I've made two very small purchases in the past from Traffic-out and the productivity was good - even on the shitty little site I was sending it to. I did make a single sale directly from it, which almost paid for one of the traffic purchases. If you had a pretty refined page with no leaks, one could make a profit. I couldn't, but someone who knows what the fuck they're doing probably could.
I think, if you had a nice link/content site that was regurlarly updated, quality purchased traffic would be a good way to gain some exposure and bookmarkers. I'd never buy blind, 404, or other types of shit traffic. Clicked traffic is good stuff though and worth exploring.