I booted up the beta for the Big Picture Mode a little bit ago, and so far it's great. The best part is that you can use a controller to maneuver about. I'm using a 360 controller, and you can just push the guide button to launch Steam into the mode. On top of all that, the most impressive part is the text-entry system they came up with for when you're using a controller. It's very innovative (at least I've never seen anything like it), and impossible to describe without a video, so I took one:
Personally, I think that's way better than scrolling through all the letters like we have to do on all the other consoles now.