I've been using it since the Beta came out about 5 months ago. Bought myself an ASUS WAVI to transmit HDMI to my big tv in the lounge room so I never have to move the computer to play, I just plug in the HDMI cable and I've got my computer setting everything automatically.
I really like Big Picture as some games are just meant for the couch and controller. I played Tomb Raider Anniversary, Sideway, Super Meat Boy, Super Crate Box, Tony Hawk Pro Skater HD and Shank with great funness. I even played a bit of CS:GO but I still prefer my FPS with keyboard/mouse.
The interface is quite sleek when you get used to it but I keep a Keyboard and Mouse handy just in case the game crashes because otherwise you're stuck when the controller can't do anything.
The web browsing interface I found is difficult to use and while the text input method is interesting (not sure if it's unique though) it takes practice to get the hang of but could make quick typing once you get the knack of it.
You also need the graphics to actually play on an HDTV at full resolution. For most people this won't be an issue or you can just play on a smaller TV, but for the big screen enthusiasts you need a card that can handle 1080i (lucky for me my card was designed for that. Yay!)
Overall, while I don't use it a lot as my girlfriend disapproves of me using both the computer and tv at the same time, it is a really good idea and I like that Valve is trying to break into the console market without costing us anything.
Awesome