I've always used the Nike+ iPod system for keeping track of my runs. I think the Nike+ iPod system (uses a sensor in your shoe) works fabulous. I have used another GPS app for tracking runs outside while also using the Nike+ just to make sure it's accurate. It always has been within .08 of a mile. So I'm happy with it and I keep using it.
When I love it most is on the treadmill. I think it accurately measures my stride on the treadmill and keeps me on track. This is important because no matter what speed I set the treadmill, I make sure my pace is calculated at around 11 min/mile by Nike+. My "easy" pace outside is currently just over 11:00 min/mile so I want keep this pace on the treadmill. Sometimes my pace per Nike+ is faster than the treadmill setting would indicate, so I'm glad I have my gadget to fall back on.
As I said, I'm Type A, love numbers and have a passion for gadgets. All this adds up to a positive relationship with Nike+ iPod, and I highly recommend it for a simple mileage/pace tracking system.
*These opinions are my own and Nike did not pay me anything.