I downloaded this a while ago through advice of a friend. Barely received any surveys, so uninstalled. I reinstalled a few months ago, when I saw he had over £6 credit from it followed by the words "I told you so". I redownloaded it, and in the last few months my balance has indeed accumulated slowly. I've probably received around £3, but this covers the cost of all the 10p apps on Google Play that appear (therefore not costing me a penny).
I don't know if this has anything to do with it, but I feel that ever since I started answering the questions honestly I get more credit. For one survey I received 47p, but I have received credit as low as 9p for the smaller surveys, or surveys where I didn't have an answer to the first question. (for example, it asked me if I'd visited a museum lately, but I was actually in the KFC nearby).
Answer the questions honestly, especially at first. The first questions you receive will seem so stupid, but its just testing if you are human. I can't remember if I got credit for the first questions (I don't think I did).
There was an old deal posted with everybody cold voting because they weren't getting responses immediately. You need to have this app installed, and just LEAVE IT INSTALLED on your phone. It will alert you when a survey becomes available, and a drag down menu with a Star icon will appear. It usually asks about locations you've visited a fortnight or so ago, mainly for feedback on your experience there.
If you answer them honestly and give constructive feedback, they come back to you with more questions. You DONT need navigation turned on, it somehow does it from data and wifi access points also.
I know its not a "deal" but there isn't really a section for this kind of thing. I think an opportunity for free appstore credit should be logged on here with more information. I'm not alone, but you need to be patient. It only takes an online Google Search/browse on Reddit to realise there are plenty of users with the same experience, so this does actually work if you do it properly.
EDIT: Here is a source for information https://www.reddit.com/r/Android/comments/22hmq3/google_opinion_rewards_just_made_me_prove_im_human/
If this miraculously makes it to 100 degrees at least more people will be aware. If you don't know about this please vote hot, as there will unfortunately be cold votes from people that already know about it.