I'd say pair programming always starts organically at my current workplace (well, it's certainly never really planned; mostly it starts with "pair debugging"). And I have to say: Given the right partner for the task, I usually really like the experience.

For me, a big part of this is that rubber ducking seems to really work well for me as well as some of my coworkers, but although I actually have a duck at my desk (more of a gag), I could never actually use it -- in an open space office, I just can't help but feel like an idiot while discussing the codebase with a toy ;-)

Of course, a big upside to a coworker playing the duck part (aside from them not being as stoic and giving me valuable feedback) is that this practice automatically familiarizes more people with a given piece of code, which is also never a bad idea.