Coming from Rich at IGN...
One room - about 18 feet - away from Wii U and had a couple signal issues with GamePad. Nothing huge - though it cost me a boss fight. For the most part the signal held but latency hit a couple times - whole picture skipped a few frames. Played for hours though. Not bad. Just reinforces the fact that this isn't a portable, really. It's a controller for a home console - and it has a screen.
As I've said many times, the distance you can use the GamePad from is really going to depend on your home setup. The more walls between you can the Wii U, the less likely it'll work properly.