There are several sites of pay-per-click, is all of these are reliable?
It depends upon conversion optimization of ads.
Most of the time, you will find ads on parked pages through online advertising agencies. Companies often have some say on which sites they want their ads to appear.
PPC is a fast way to increase traffic,so I think it is good and credible ,but sometimes more traffice not equal more incomes.
For pay per click, I would say that only use Adsense and Adwords of Google.com they are too popular now.