- Unlike most other projects I've worked on, I've seen an overall improvement in the code quality over the past few years. I've never seen that happen on any other project. As people learn better ways of doing things, pairing helps to disseminate that knowledge in a way that meetings, presentations, and code reviews just don't.
- In my experience, there hasn't been a problem with people who are working together arguing and consequently getting nothing done. On the contrary, pairing seems to have encouraged an atmosphere of cooperation and friendliness.
- Sometimes under time pressure one can't resist the urge to copy and paste some code or hack in some functionality without developing tests first. The mutual supervision of pair programming really does seem to have a positive effect on these kinds of transgressions. It's a lot harder to take a nasty shortcut under the watchful eye of the person you're pairing with.
- I would tend to agree with the principle that working in pairs doesn't really hurt overall productivity. The reason seems to be that the team having a common understanding of the code and the business generally trumps the value of having two people typing code in separately. Integration is the real difficulty in many software projects, so whatever gain in the amount of code written might come from separating pairs, that gain seems to be offset by the greater consistency of code written when pairing.
- Hygiene: Pair programming in our environment means sharing one keyboard and mouse, and sitting in close proximity to another person all day long as well as switching pairs frequently. We often have had problems with people spreading colds around the office. I think developers in an XP shop should have their own personal wireless mouse/keyboard combination as well as their own chair that they don't have to share with anyone else.
- Ergonomics: The reality is that sitting at a computer is not a natural position for the body and you can do damage to your back, neck, shoulders and head, and of course your wrists over time. Pairing tends to encourage bad ergonomic habits, because it's inconvenient to change the position of the keyboard tray, monitor, chair, etc when it's time to take one's turn at the keyboard. I really believe that these things need to be considered if you want to do pair programming long term.
- Personal space: Some people enjoy having some peace and quiet as well as a space where they can keep their things. As much as I am a fan of pair programming, I think developers should have their own desks away from the common bull pen they can retreat to from time to time.