I think it's best to stay away from Miva because all the fraud clicks you get. Customer support isn't great and sometimes it takes them forever to get back with you. I would recommend staying with Yahoo! and Google's PPC. However, you'll never know if it's good for you unless you try it, so give it a shot and see if it works out for you.
In my personal experience Miva shouldn't be relied upon for sole paid search marketing, but it can provide an effective supplementary function. If you have already used G and OVT for ppc, you may find constructing campaigns within Mivas ad centre majorly infuriating. The user friendliness is pretty dire - if you want to change a bunch of bids for different keywords for example, you have to visit each one induvidually. But I'd say give it a go, you can only try with these things..