I've got 3 months free All Access with a Chromecast. I'm quite impressed with the radio feature in GPM. It feels quite like Last.fm radio was a few years ago. Some minor irritations and comments though.
1) Sound like radio for a single artist is surprisingly good.
2) The Genres are too big and too few. I really miss last.fm's crowd sourced tags and then being able to play "Tag Radio". Something like "Cocktails at Sunset" is a lot finer grained than "Dubstep". Music is too big and complex to be able to reliably reduce it to just 20 or so genres.
3) I miss all the community features and especially "Friends Radio". It doesn't feel like there's much support for saying "hey, listen to this" to your friends but maybe I missed that.
4) I kind of feel it should be better at working out my tastes based on my library. That's probably going to take a bit more digging and use to confirm.
5) The video linkage to Youtube and Music Key feels primitive at best. I'm not sure how Google is going to merge these two things.
6) I'd like to have seen Google work better with third parties. But I think this looks to me like they're trying to blow them away long term.
It doesn't feel like there's enough there for £9.99/month for me. I've already cancelled so I don't forget and I doubt I'll change my mind. The question is what I would pay and the jury's out. I've become so used to free music, I really doubt I'll go back to paying something.