Just tried it. My experience might be abnormal, but it seems glitchy and slow. A playlist that I already created with my offline songs synced with my phone, but playlist said 0 songs. No error messages, very unintuitive. Had to create a new playlist and add the songs again, still wasn't showing. Relaunched spotify on both devices two times and it finally showed up. And this is for ~20 songs. I wouldn't want to use this method on a big library.
Also, Spotify doesn't upload your music for streaming, its just a sync between your phone and computer that have to be on the same wifi, and keeps the songs downloaded on your phone. Wouldn't work for a collection bigger than your phones storage. I wouldn't even compare this to GPM.
Overall I’m very happy with Spotify but experience has shown that within a few years it’ll begin the slow and unstoppable turn towards Villainy.