I'm using squarehome.tablet - would love to try ARC Browser by Floating Point, a gaming front-end like Gamesome and Hyperspin, but they are currenly focusing on Android TV. Politely wrote them asking if I could try on a portable device, and they politely told me no.
Another worthwhile front-end in the OUYA store is Nostalgia, which you can load via the OUYA loader in the app store, but you cant pay the dollar to unlock full functionality. The author is working on a Play Store general Android version, but his progress is VERY slow...