I use all 3 PPC. I use Adwords, AdCenter and YSM. On my very own personal opinion I rank YSM as the lowest performing, the slowest and the more intrusive one (even with the adwords quality S###). I will rank AdCenter as # 2. Their interface is awesome result are pretty acceptable and research tools are good to. As YSM AdCenter is kind of slow. And for number one my best performer and easy of use is Adwords. And for speed it may go a bit slower some times but it will defenetely out rank YSF and AdCenter. I will like to know how you guys rank them.
After using YSM (i.e., struggling and cursing), I finally signed up for AdWords, and what a breath of fresh air that was! I don't know about performance yet, but G's interface is about 100,000x easier to use than YSM. YSM's interface is antiquated, slow, and intrusive. Don't know about AdCenter, though.
I like Yahoo, they sends me quality traffic , Big G adwords is too expensive for me, tried for few months gave up