During the last few months I've been using an ads system called Pontiflex.
This method asks for name, surname and email and offers users the possibility to register in order to receive info from the ads they choose. Due to this ad being an initial popup, it wasn't very welcome.
This week, I've tried Admob, by Google, whose ads are a lot less intrusive and I've received some better marks.
On the other hand, Admob produced 4$ this week. I hope you all understand that Admob's are useless unless it gets to 10k daily impressions. I barely reach 1k.
So, on the next update, I'll try and use the Pontiflex ads. Maybe adding some sort of interval (i.e. appear once every two app startups).
In the end, with Pontiflex lots of users gave me bad marks WITHOUT ANY KIND of feedback. I'm not aware of any errors, their specs or anything similar. It seems like some of them see the ad more than once every time they launch the app, but I've failed to confirm it since they don't get into my site or mail me. Having tested this app on 3 devices, I couldn't find anything going wrong, but who knows...