I learned with Google music never get involved with Google with something you'll want to use daily. Google music hands down was the absolute best music service I've ever used. Google is like a kid with ADHD bouncing around from project to project never to see them through.
I used it from the start, I got in on the beta, and while it was nice, I wouldn't say "it was the absolute best". IMO Spotify is just as good, in fact I'd say it's better. It was nice that you could upload 20,000 of your own songs, but that was back before we had hundreds of gigs available on our phones.
For me it was the ability to upload my own library and stream it without a subscription, I ended up switching to plex and running my own server for a while but yeah Spotify just has the best deal with no effort so I caved to that especially with a family plan you can't beat it.
IIRC Play Music was a paid service, it probably had a free tier with ads/commercials, but I definitely paid for it. That's how I got grandfathered into YouTube Premium (when they launched YouTube Red as it was called it came "free" with the subscription to Play Music) and can't go back to the free tier or deal with the various hacked clients and ad blockers. It's been over a decade of ad-free YouTube (except for the sponsor segments everyone does now, I do use SmartTube on my Nvidia Shield to skip those).
As a former Google Play Music user and lover, it was the recommendations. I haven't found another service that shows me even a significant fraction of the music I like that Google did. I've switched to Spotify but it constantly recommends songs I've already heard or don't like and the shuffle feature gives me the same ~50 songs from a large playlist. It's something I've accepted but I miss the Google recommendations deeply.
I'm the opposite, I never really used GPM's recommendations because I knew what I liked and had tons of ripped CDs in my collection. After we all ditched MP3s and went to streaming I still stuck with what I knew. I only switched to Spotify about 2 years ago and it has opened me up to a bunch of smaller artists in Europe (I'm in the US) that I would have never found on my own. One of them (Green Lads) I've listened to for 2500 hours this past year thanks to their recommendations.