I did it for two years. I found that it was draining to be focused the full time every day. I liked the way it worked, I loved the code quality that came out of it, but I can't do it 8 hours a day month after month.
I was exhausted the first month, but... I actually hate myself when I slack off. I love few things more than getting things done, and pairing provides me with that euphoria and discipline.