Thing is, what to spend that credit on? There aren't a lot of apps that I find especially useful or needful, and most of those apps are "free" or, at most, "freemium." When it comes to music, I already have e.g. Amazon Music Unlimited, "free" Pandora and Spotify, plus of course YouTube. Not to mention my own small vinyl collection and Tamara's extensive CD library.
While I also maintain a reasonably large library of dead-tree books and a growing library of Kindle e-books, I figured I'd check out Google's book offerings.
Hey, an offer for $4 off any book priced at over $X!
And Thomas Pynchon's Bleeding Edge was priced at an insane(ly high -- I think of $5.99 as the highest reasonable price for an e-book) $12.99. Minus four bucks, that's $8.99, so it should end up costing me less than a dollar.
Except that Google Play didn't apply my Google Play credit, just the $4 discount. It charged me (via Google Pay) $8.99. Which is three bucks more than I was really interested in paying for it, though I'm glad to have it.
So caveat emptor to 1) Google Survey respondents who also 2) use Google Play 3) to get books. I'm not sure if you can't use Google Play credit for Google Play Books at all, or if you just can't use it in conjunction with a deal.