I had been burned by these services. I think that they can be effective if used right, but in general can be dangerous. It is easy to get carried away with the visits, and end up having to pay a fat bill when you only get a sign up or two out of it.
I said this thousand o times, and I say this again.
I don't think people honestly realize how big of a scam overture and google can be as well.
USE ROI TRACKING TOOLS and ANALYZE your stats.
Some of the smaller PPC's may actually do better since they can offer a more personal service.
Your PPC campaign is only as good as YOU make it. If you work hard and understand the networks you are advertising on then YOU can make it succeed. If however you don't trust the arrangements before hand you won't trust your results and you won't put forth the effort you need to make it work.
It isn't up to the network to make things work, it is up to YOU understanding YOUR audience and correctly targeting to them and TRACKING your results.
Doesn't matter if its google, overture or anyone.