At one time in my career I pair programmed pretty much all day, every day for the better part of four years, with the same person. We started with an existing codebase, but even from the start most of the work we did was on new features.

It is intense, but--at least in my experience--it was also very productive. You basically can't/don't ever slack off. And I learned a lot from the other developer (mainly about various tools). Things that theoretically I could have learned on my own but in practice I probably never would have.

If I had it to do over again I would definitely choose to do it again.