I've used pairs off and on at work but never long-term. The part I find most useful about coding side-by-side with someone is that there is someone to question (or reassure) every small decision I make (for better or worse!). Particularly in coding, there are always many ways to solve each problem and having two sets of eyes (with even mediocre brains attached) frequently yields a better solution.

I find that a necessary part of pair programming is to switch up the pairs fairly often. Pairs have a tendency to become "comfortable" with each other's habits and can fall into a rut -- pairs should be shaken up to keep each everyone on their toes.

Also, you can learn a lot (about programming, people, life, etc.) when pair programming.