I use my IPOD Touch Generation 2.0 for a lot of things, included running. I purchased the Nike Sensor to add Mizuno Wave Creations running shoes. Every time I run, it records my distance and pace. I have found it amazingly accurate.
Since Nike records all of my runs and sends it to its website, it is very easy to track your distance and times to compete with other people. This is great motivation to get in shape and keep in touch with friends that have moved far away.