Lots and lots of people use Google Play Music. How many? 500 million, give or take a few. Google's music player shot past that mark just recently to join the likes of Google Chrome, Hangouts, and Google+ in the 500+ million club.
Also: Hey Google, where's your developer address?
Google and Facebook (post Whatsapp acquisition) remain the only developers in the Play Store that have managed to reach the 500 million download threshold. However, several of Google's apps (like Gmail and YouTube) have pushed past the 500,000,000-1,000,000,000 bracket to reach over a billion human beings.
Play Music's feature set is still restricted by country, but it has expanded dramatically from the original US launch. It's not a perfect app, but do you remember how terrible the stock Android music app used to be? I continue to have reservations about those deauthorization limits, but I still use it.