I've done plenty of pair programming informally when one of us were stuck on some hairy bit of code and needed another pair of eyes and some extra brain power to get to a solution but I've never done it in an ongoing formal context. What I've always wondered is if you get the doubling of productivity required to justify having two resources working on the same bit of code? Certainly in blocking situations you can get a huge multiplier on productivity (I'm including quality and correctness in my definition of productivity) but lots of code is mundane stuff that just needs someone to push through and knock it out. Can a pair get the mundane stuff done in half the time of a solo developer?