I have been using various contextual ad networks to promote one of my sites. It's difficult to judge results accurately, since if you're getting search engine traffic as well as paying for clicks you don't know exactly where most of your leads are coming from. That said, you can see some trends emerging, and make reasonable conclusions from them.
So, from what I can tell, Clicksor seems to be one of the better ones. Bidvertiser works okay. Adtoll is a smaller network, and delivers hits much more slowly. But it seems to deliver good traffic. I will use all of these sites again.
I have used "run of network" ads with Adengage, but the clicks were delivered super fast and I didn't get many signups from them at all. I won't use this site again.
I've just started with Adwords. That's more involved and I'm still getting my head around it but it appears to be working very well. However, to get clicks you have to pay quite a bit more than you do with the other networks I've mentioned. But those clicks do seem to be much more likely to convert than all the others. So, definitely worth it.