The biggest issue with Big Picture mode is that it does nothing to help with multi-monitor setups. I have a 42" HDTV hooked up as a secondary monitor but it's too much effort changing the default monitor, then changing the in-game resolution and changing over my default sound system to HDMI. What's needed is for Big Picture mode to run on whichever screen it is launched from and make all games run on that screen - as it is even if you launch Big Picture mode from a secondary display it will launch on the primary display.
Big Picture mode is a nice idea but it's useless for me, and that's despite having a wireless X360 controller and a HDTV connected to my computer. It really is niche functionality, which is obviously just to test the interface for their upcoming hardware / console.