iOS: Google finally launched the iOS version its Play Music app today. The app includes support for playing your entire music library, as well as any songs from Google's library if you have the All Access subscription.
The catch is that you can't buy music directly from the app, but you're still able to buy individual tracks online and stream or download them to your iOS device once they're added to your library. It's a nuisance, but you at least have access to all of the tracks you would on any other platform.
The app also lacks the "I'm feeling lucky" radio station, but it otherwise has near feature parity with the Android version.
Google Play Music [iTunes App Store]