I'm testing out Adcenter and Yahoo right now.
Obviously, on Adcenter you're going to get an older demographic. These people don't even know how to get to Google. However, my CTR has been really good there. The problem though has been the amount of traffic I'm getting there. Bing just isn't going to send you a lot of stuff unless you are in a competitive niche that targets older people. Then your CPC will probably be fairly high. I haven't been able to get super cheap clicks. I'm down around $0.19, not great, not horrible.
As for Yahoo, the amount of traffic I'm getting is far superior. CPC is pretty good. It is fairly easy to setup, although the "Advanced" vs "Standard" threw me off originally. Make sure you research that. "Advanced" tends to be the broad term approach, which I haven't really tested out. Good luck!