So how accurate is the Nike Sport Kit? When I first tested it in a two mile run, the iPod had me run about 7% longer than the two mile mark. Then I calibrated it using a 400 meter running track. During my next run, the iPod had me run about 2% shorter than the actual distance. I suppose that is accurate enough to be useful. However, when I ran a fast two miles (with a faster pace than my long-distance running speed), the iPod once again had me run about 6.5% farther than I should have. This is probably because my strides were longer, which made the iPod think I wasn't running as far.
So, the Nike Sport Kit can be pretty accurate, but only if you calibrate it and maintain roughly the same running pace for your runs. It is certainly better than guessing your distance and the vocal feedback is a helpful feature for any runner.