Well that pretty much sums up the way that I feel about it. If you already have a Nano, then it's a pretty small investment for such a cool toy. It is so simple really but somehow it really motivates me. I do use it on the treadmill but did find that it was a bit off on the distance. I haven't yet calibrated it but it seems much more accurate outside.
I love setting a distance goal and then hearing my progress. Their website is also really cool and simple and it can track all your runs. I love going home after a run and plugging in the Nano to my computer.
Simple and fun!