I haven't used it but I have heard about it from my friends and also read reviews from internet. I am using Adsense and pretty much satisfied with it. YPN is also better option for PPC.
It is a complete failure!! I have used it and guys it scared me so much that I couldn't sleep for two nights. lol Actually they are doing so bad that people have decided not to use it.
It's not that good. I tried it for a bit and saw the same, sometimes less, results as AdWords. I'd just stick with AdWords. They give way better results and it's worth it.
Adcenter!!! rubbish!! Just a wastage of time and nothing else. I used it and was scared with the service they provided. Google Adsense is way ahead and far better then Adcenter.