Hm, imho, if *I* was spending X per month with a company, I would really want to know that I was getting the value for that X / month spend.
Without the right kind of closed loop tracking system, you really can't know if the spend is working or not for you.
If you have that kind of system in place, monitor the results, and if you are spending on *any* PPC system, I would recommend that you experiment with all the major players:
And then, throw the XML / CPC programs (if you can) in the mix, as well as the paid inclusion with Yahoo.
Track the results, and see which ones give the best ROI / and best customer service, etc.
Vote with your wallet, and the market place will correct itself. :) Good luck - and let us know if / when they get back to you - it's very helpful to the rest of us to know how to allocate our $$$, because sometimes, it's not just about ROI, but also about service, too.