Compared Bards Tale on my Nexus 4 (rooted to get full screen via video out and usage to a PS3 controller) to the OUYA. Nexus 4 wins. Surprising amount of apps support multiple Bluetooth control options (if you are willing to tinker with your phone), and most (android) phones have a video out option now. I really want to love OUYA, I really do, but after fiddling with it for a week, it's just not doing what I wanted it to do, and was expecting from it.
What was it you were expecting? It sounded like OUYA was pretty clear with their platform and capabilities from the get-go.
Of course the N4 wins--the Nexus 4's CPU and GPU are superior to the T3 in the OUYA.
One advantage over using a phone is that it's a giant pain to connect power, video, and and external controller to a mobile device like a phone. Plus, and especially with modified Android, the confidence in a good user experience goes out the window.