I have been using them since early 2006.

I had three problems with Clicksor:

1. They pay bloody awful rates.

2. 99% of the time the ads have absolutely nothing to do with your site content.

3. They are only too willing to allow spyware sellers to advertise on thei network.

several times I have had to send them stern emails complaining about spyware sites hijacking my site and forcing my visitors to go to their site where spyware will be installed with or without their permission.

I currently use Adbrite who seem to have a better pay rate.