I bought the new (at the time) second generation Nano back in Oct. 06 and bought the Nike+ kit the same day. I've ran over 270 days this year and all of them have been with the Nike+.
I can't believe all the reviews talking about how bad the battery life is. Not only that but they mention following the instructions on how to maximize said battery life. In a year I've not done one thing to my sensor except put it in two newer pairs of shoes. I never take it out. I never turn it off. It's in my shoes through -5º days, it's in there in 95º days. It's been through rain, sleet, snow and not once have I even thought about removing it to dry out.
Finally a year later it's time for a new one. For about a week I knew I was running faster/farther than the data showed. Sure enough today I received a message on the Nano stating my sensor battery was low and needed replacement. Not bad.
I think I can pony up for a new sensor yearly. Especially since I've found you only need to buy the sensor and not the whole kit.