Pros
Pair programming is a great way to learn from others, I left better than I came in. XP process taught me a lot about delivering incremental value rather than releasing lots of code. Lots of free time to learn and experiment as you get 20% learning time. Lots of personal investment as you get a conference budget you often struggle to use up. Because of the pairing you never work long hours.
Cons
Requirements and focus often change on a weekly basis meaning you don't get much done and you often end up changing what you wrote just a short while ago. A lot of technical debt in older code that you need to change and support means it often take a week to do what should take half a day. Not much room to change the developer culture as older developers who have been in the company longer will always have a stronger voice than new people. Everyone is expected to be a generalist, whether they want to or not and wether it's good for the product or not. Pairing often prevents developers from being entrepreneurial and independent.