At work we use pair programming all the time.
The trick is to know which tasks should be done in pair and which would be a "waste of time" if done by two developers.
The rule of the thumb is that tasks that are more research oriented (i.e. POCs & spikes) should be done in pairs as well as development of new features (so that the knowledge will exist in more than one mind).
Tasks that are more mundain such as CI server installation or replacement of the addon icons is done by a single developer.
Another factor is the current availability of team members and the current tasks to be done at that iteration.
