We used MdotM which offers a CPI based model. It was certainly more effective than Admob although still hard to tell what the long term value of downloaders was once on board and no real targeting offered.
On our end we decided to create our own cross-promotion network to chain our apps together and 3rd parties as well as we reach scale. Its a free 1-1 download trader, a paid layer may be created down the road. I'll let you know how it goes as the first sets of apps get on board. Completely agree that trading or buying downloads (quality downloads vs fluff) is the next wave.
cheers, Michael
Get Splash!