Pair programming reduces variability in productivity rate, so looking back on a pairing session you should expect to remember occasions when you felt that you'd been slowed down, and should not expect to have noticed occasions when you'd saved time.

Alistair Cockburn has a study up somewhere showing (from memory) a small increase in work hours to first thunking you're finished, a large reduction in bugs, and an improvement in subjective measures if code quality. Trike's experience matches this study - we think pairing is awesome. .