As somebody working in that market, honestly most music services look the same to me and if you are fully satisfied with whatever music service you use, you probably don't really need to switch.
That being said Play Music is pretty great for me because :
-It lets me upload my own mp3s without needing anything else than the folder where it needs to look for them. You can even ask the app to always monitor a specific folder for new music files. It tries to match these files with its catalogs and if they match they will be automatically added to your library instead of being uploaded. When you upload an unknown file, you can alter its tags and cover if you in Play Music and you can regroup them in Albums anyway you want.
As somebody with a huge existing music files collection, many of which are not available on any service, this is a killer feature for me. I understand that it is not going to appeal to everybody though.
- The Android and desktop apps have been constantly great for quite some time. The only complaints I could form is that this orange is not really great and that on Android they should probably get rid of the Transitions. The design idea is great but the platform does not support them really well. I have seen some kinks in the app recently but they might have been caused by the Android N beta build I was using at the time.
- Most of the big music services have the same base features : albums, curated playlists, automatic radios, family plans ... Play Music has these as well for quite some time. The suggestion algorithm has been pretty good for me but that's probably very subjective. Play Music also has nice contextual suggestions based on their Songza acquisition (for example right now : it is sunday afternoon, it proposes running, pokemon, BBQ, beach, ... musics ) .
- So overall it is a great service, IMO absolutely on par with Spotify or Apple Music. The price is the same as any other music service .. but you also get 10% off on all of your Play Store apps & musics purchases and if you are in the USA, it also gives you Youtube Red (= no ads on youtube). The value proposition seems pretty insane to me compared to the other music services.