I have an old 40G model, generation 3 I think? It's battery died a few years ago already. I don't know its age but at least 4-5 years I think. I replaced the battery as well but it didn't work too well...it won't hold the charge properly. If I charge it fully, it'll last an hour tops, and sometimes it dies even if it's in the charger.
But, I keep in in the car, charging all the time, and it works well as a car audio source.
I also have a 120G which is barely a year old and naturally still strong battery etc.
Overall, I guess today most any piece of equipment won't last longer than three to five years. My first digital cam is roughly five and on its last legs, had to buy another already. And the DV cam is 4,5 years and hiccups quite regularly but still more or less works (been to maintenance once already) but it looks like any day it's time for a new video camera as well.