I once did about 3 months of pair programming with my boss/senior developer at the start of a new job and I can say that it gets old pretty quickly, the comments about talking all day and being driven crazy when someone else is controlling the screen are spot on. But you do pick up mistakes quicker.

A balanced approach is great, a mix of paired work, and solo work with code reviews is better. Also if you are going to work in a pair you really need to have a room to yourself. I imagine it would be a nightmare in an open plan office with dozen of pairs going at it (so to speak!) at once.