I recently bought a Nike+ GPS Sportswatch (Dec 2012) and started to see some irregular results in my data - as much as 20% of the distance. I started to investigate.
Not only was the distance way off what other peoples Garmins reported, their website 'data' seemed to be more accurate than their website stats they were reporting.
Note that I had applied all firmware and software updates to the watch / computer, and also did all runs using the foot pod.
Find below two links, one is to the original investigation I did, with results, and the other to a Nike GPS testing tool I've wrote. Hope it helps shed some light onto the situation.