I've been pair-programming for over 15 years now, and was introduced to mobbing by Woody and Llewellyn a couple of years ago. I've tried it in a couple of teams for specific tasks. I don't think that in a team of experienced devs I would mob all the time (although if the team wanted to I wouldn't veto it), but it definitely has its place. In particular for learning new techniques, large refactorings and exploring solutions with customers.