If you already have an Amazon Prime subscription for the other perks (I don't, but know several people that do) then the Amazon solution is a nice bonus and allows you to enjoy ad-free offline music - great for the commute without burning up data.
I haven't updated the amazon MP3 app (which I used for my music on my devices). It's been 'meh' up until now, and the reviews in the google play store indicate that it complicates accessing your own music files. That being said, I think I'll migrate to google music. We have Prime, but I have been using Slacker radio for quite a few years, and I like its music options, stations, and customizability.
I just want a decent radio algorithm. Spotify is trash for finding songs that actually fit into the genre I like, pandora is better but still endlessly repetetive, playing maybe 30 songs I pressed the like button on for every 1 new one I haven't heard on the station yet and like
One thing certain, this situation will not last long. Amazon knows how to listen to it's customers and you can bet that whatever shortcomings the current music system may currently have, they will be corrected over time. Amazon may never best Spotify, just as it hasn't bested Netflix or the other video providers, but it will become more and more competitive.