Ignoring how click-baity this title is, lol, I can attest to this being 100% legit as this is how I’ve purchased a lot of games from the Google Play Store.
One thing to note is that this app pairs best if you keep your phone’s location on as Google will send you more surveys depending on the places you visit. As far as I can tell, this is to aid in increasing the accuracy of Google Maps, and is also entirely optional. Keep in mind though, if you opt out of that—at least in my experience—you’ll receive less surveys because Google just has less info on you telling them that you would be an appropriate candidate for such-and-such quiz.
I’ll preemptively admit that this can all come across as creepy as heck, given that you’re effectively giving Google information regarding your habits—I especially get uncomfortable when I get surveys following my use of Google Assistant—but considering how much information Google has on me anyway (via Google Maps, Google wallet, my personal gmail account, my school gmail account, etc)…meh. I figure that I should at least get something out of it, you know?
And this is just my opinion of course. The Opinion Rewards app isn’t a requirement to purchase items through the CoG/HG/HC omnibuses (omnibusii?) or even the Google Play Store. This is entirely supplementary…gosh, you can’t even call it income because you can’t actually exchange it for money, so you’re effectively just receiving Google Play Store credits, I guess? lol
The point is, it’s ok if it’s not something you’re comfortable with or interested in! To each their own!