Hi Mike, I've used findwhat/miva and a couple of the other ones before.
I don't think the reach (number of people that see it) and technology is there really for the smaller ppc ad networks.
Sometimes you might find a good buy, but I think overall you'll get the best results from the top tier networks. Here's a good thread at WebmasterWorld that answers that exact question:
http://www.webmasterworld.com/forum85/1142.htm
Microsoft has recently launched their own PPC advertising that is supposed to be pretty good:
http://advertising.msn.com/msn-adcenter