I use a Nexus 5 and iPhone 5S and subscribe to Google All Access. I've been using GMusic 2 on my iPhone and that works okay, but I'm really happy Google Play Music is finally here.
So far, it seems the iOS music is missing Genre Radio Stations. On Android I can go into the Genre section and start a radio station (a new feature added to the Android version not long ago). The iOS version still doesn't have that, so seems to be a bit behind (hopefully it won't always be half a year behind the Android version).
Also on the iOS version if the music gets paused because I unplug the headphones, and then later go back into the app, it'll automatically start playing again (with or without headphones plugged in). This is kinda annoying as it shouldn't just start playing without me pressing play like the Android version.
Other than that, it seems to work pretty well. Yeah, it's not the best music player out there, but I don't think most other music apps will support streaming your All Access music subscription.