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But... The market leaders in PPC (I think it is fair to say) are Google and Overture. The one you mentioned (on their own site) claims to be ranked 179. There are a number of websites that give opinions and ratings of all pay per click suppliers, but usually their strength depends on the power of their partners. Ask yourself if their results appear on third party sites of "substance"?
Of course, if you want cheap traffic then the smaller PPCs are great. But then ask yourself "Am I the only person going to ever click on my result?" and if not, where are the real users coming from.
Having a tracking system that allows you to track sales back to original inbound links makes these sorts of questions much easier to answer.
I wouldn't bother joining this one