Jason Snell wrote a piece on why he writes on his iPad. From the article:

The iPad also offers a remarkable lack of distractions. When I write on my Mac I find I am endlessly checking Twitter and email and my weather station’s current conditions page and anything else I can find to distract myself from the difficult task of putting one word in front of another. On the iPad, I am more focused—and when I do finally take a break to check my email, it feels like an actual break, not a distraction.

He nails it with that quote. I find that writing on my iPad—while not as fast—is very much a distraction-free experience. It also explains why full screen writing apps like WriteRoom are popular.