If it's just like Spotify, it will be a dud. People already know Spotify and what they do, and by and large they're really happy with it.
Google is doing it for the wrong reasons. They believe that iTunes is a big source of the iPhone's power with consumers, and that they need a service that can do all that iTunes can and more and they'll win smartphones.
That's why they did Google Music; basically a music-only DropBox (uhum, Google Drive, then). Then Apple promptly trumped them with iTunes Match, and they had to start the photocopiers running again. Google's initial solution was dumb. Apple's solution seemed to 'get' music listeners a bit better.
Apple do seem to get music. Google just wants Android to gain a Windows-like dominance of smartphones so badly they come up with all kinds of dumb, unimaginative solutions.
That won't be all that's announced at I/O, though. There's a Google Maps redesign coming that looks
eerily similar to Apple's Maps.