A major practical & eco problem with Nike+iPod kit is the sensor battery, which runs out typically within a year. It is not officially user replaceable, which does not mean anything really.
Take one: used Mora-knife and a small hammer to cut the sensor open. Some small plastic parts flew away. Not a prob. Next used a small screwdriver to chop the battery out. The model number turns out to be CR2032. It’s an ubiquitous one you can buy from any electronics / tech / camera shop for 1-5 euros…