For an explanation of Pair Programming, please visit this wiki entry.
Good
I am pair programming on the ScrumPad project now and the feeling in general is good. I have this perception is the work quality is better than doing alone. Also, the amount of time wasted to find small issues are now much reduced. Overall, its good.
Productivity Increased
I definitely found the productivity boosted by a factor of 1.5 at least. May be this is specific to my case, as I am working on a mid scale refactoring task. We worked on two user stories so far and completed the stories in 1.5 times faster compared to the individual estimates. We are doing estimation on a regular basis and the variance between estimation and actual hours are less than 5% as we see in the last 17 sprints in this project.
Disclaimer
I am yet new to this practice and not in a position to draw a conclusion yet. However, I believe our open space set-up and strong team culture helped us a lot to start this pair programming.
Are you pairing? Please, share your feelings and suggestions.