I've used Nike+ for nearly 3 and a half years to track my runs. It was the best Christmas present of 2007. I was sold after using it twice. It was reasonably accurate out of the box. It gave me extra motivation and accountability. I ran with my iPod anyway. It had a slick website with a mini me. The more I ran, the more active the mini me was. I can go back and look at all of my runs since January 2008.
As much as I have gotten tremendous value out of Nike+, I just got a Garmin 305. Reasonably accurate wasn't cutting it anymore. I calibrated my Nike+ sensor recently, which improved the accuracy a lot. There was a very small margin of error (2%) between Nike+ and Garmin distances when I took both devices out on the same run. But I think I have outgrown Nike+. The Garmin is better for where I want to go with my training/running. Accuracy aside, the Garmin provides so much more data. Such as mile splits, elevation, maps, and more info on pace. The display is bigger and can be customized to show the data I want. My not-so-secret inner data diva is pleased.
Oh, I will not completely abandon Nike+. I alternate between the treadmill and outside, so I will still use it on the treadmill. Although Garmin's website is more useful, Nike+'s website is more fun. There is a way to add Garmin runs to the Nike+ website (not automatic but fairly easy to export the Garmin run, convert it with a website, and upload it to Nike+), plus my running history is on Nike+. And so I will continue to track my runs on Nike+.