I haven't used it yet (I will), but it sounds really neat. I don't like sitting here with my laptop on my lap, having to squint and curse due to the size of my tiny 1080p monitor..
I'd prefer to be scrawled out on my couch playing on my big TV screen, with a controller! That's just the proper traditional experience for me.
But then I lose the HD aspect of my screen...not worth it to me :3
So I tried it, and it's a bit of a hassle with the controls of it. It also tended to seize up on a random screen and I'd have to wait for it to unfreeze. I'd probably wait for Steam to release further bug fixes than use it at the moment.
I've had my desktop on the main floor of our house since September for various reasons so I was happy to try out Big Picture. It's alright. Most of the games I play, though, are keyboard and mouse games and even if they have controller settings, they just don't play as well. So I never really remember to go into Big Picture mode when playing on the TV if I'm just gonna be playing with keyboard/mouse anyway. :s It's a great idea though and I like that it's been implemented.
I haven't tried it yet, but I hope to at some point, because I would love to play some of my favorite PC games on an HDTV.
According to my friend who tested it, it can greatly improve performance if you are trying to play games on a bad computer or laptop. He had difficulty playing Primal Carnage without lagging, even on the lowest settings, however when he played it through Steam on his HDTV, he experienced no problems and was able to play on the highest settings.