I buy games and ad-free versions of things if I find them useful. Even if it's not something I'll use every day, I'll throw a dollar or two at them if it was a fun game or gave me some good wallpapers or whatever. It's free money as far as I'm concerned lol
I did use it to buy Nine (Exchange email client). It was $10, most pricey app I've bought, and well worth it. Still was $0 out of pocket. Love dem surveys.
I've earned $80.58 since 11/6/13 and currently have $6.86 available
You can also use it for IAP in games, buy/rent movies, buy music on google play if you use that, etc.
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If you plan on using it to buy .99 Nova Prime, good luck. The surveys are random, the amount they give is random, and Prime is only on sale to January. It may work out, or you may not get any surveys before it goes on sale, who knows lol