That's my first reaction to it too. I assume the reason it works is that when you have a mental block about something, your partner doesn't, and, hopefully, vise-versa.

Of course, if you lay down certain rules right from the start, it could work out just fine. ie - "You comment it - I'll code it." :)

Hmm... I guessed there'd be some horror stories about XP out there, and found these ones... (The articles they're based on are in the "Case Against" link on the page.)

From that, I gather developement's fast because the developers are in such a hurry to get away from the "pair" they're sitting beside!

(I'm just an amateur programmer and so can sit on the sidelines and laugh at this kind of thing without any fear it might happen to me;)