Not really. The setup is complicated and fragile, the hardware is bulky and annoying. Some games can't really work with mouse mapped to analogs. Not everything can be done with just a controller (OS updates, etc.)
It's not great, but a Linux machine with Steam on it could be.
I on the other hand, have done it. As expected, I use the computer more for movies than games (girlfriend demands). The gamepad is actually a very good Media Center controller.
The only real issue is passwords. But you don't have to enter them all the time.
My point is: having to get up and use a keyboard from time to time doesn't kill the experience of gaming in a computer and a big TV.
I want the Steam box because of the possibility of better games and a general MS distrust, not because the actual experience is something totally unmanageable, as some people here try to assume.
Most people do not like to have a PC in their living room.
PC gaming can be done from a couch but the entire form factor / experience traditionally has not been optimized for that. Thankfully it appears as though Valve is working on changing that, but there is a lot of work left to be done.
Even old games, just use Logitech controller mapper to convert the gamepad input to simulated mouse + keyboard.
Honesty that old complaint is very tired now.