I want to share my statistics from polfish from July (9 days). So maybe these numbers will help people to decide.
I have it only in one long session app, with ~2000 downloads a day (for last two days it was 2800+).
Sessions (how many times my app was used): 220 000
Impressions (surveys served): 6500
Surveys completed: 168
Daily revenue: ~5.65$
Revenue: 50.9$
Revenue per completed survey: ~0.30$ (0.3029$)
The numbers are not amazing, but the fact, that you can use it together with your existing ads makes it interesting. Because it is in addition to your revenue from ads. They started to get much more surveys, than it was last month, so probably they growing, which is nice.
So, in general, I would suggest you to use it, if you have a long session app.
P.S: it is actually, 9 days + 2 completed surveys + 100 served surveys from today.