They are good I think. I am not loving their interface, but they have huge amount of advertisers. Most of the ones I use pay out around 6%. I always get paid every month and I get weekly emails with "hot deals" and such things, which helps a lot in getting more conversions, if you keep changing your ads accordingly on your websites. I have emailed them a couple of times with questions, and the support/customer service is okay, but not very personal.
Personally, I dont like Commission Junction. They have a tendency to stiff affiliates and I know many horror stories where the network doesnt protect th affiliates if a merchant decides to pull the plug.
Doesnt really inspire a lot of confidence IMO. That's why try to get a direct deal with a merchant and make him pay upfront if possible. Takes more skill but you are better protected.