Results of our ad buys on Daring Fireball and The Deck
paperlabs.com
paperlabs.com
I'll bet you get a lot fewer fraudulent clicks and impressions (or as Google would say "Invalid Clicks") than you would using Google mobile or regular Google Display Network ads. The publishers of the sites you're advertising on aren't really incented to pump the numbers when they're all on flat rate. Either they have the influence or they don't.
We didn't have the infrastructure to support per-ad conversion/download tracking, hopefully we will for our next big spend.
It is a very pretty looking polished app. I'm not sure I'd spend money to buy it myself, but maybe others would. I suspect it will take a long time to make back the $15k in advertising.