After four years, eight months and 108,000 miles, my Prius battery pack, has one bad cell.
I bought it in early 2010, with 93,300 miles, at a sub-KBB wholesale price at a local Toyota dealer. Because of the high miles, they couldn't sell it as a certified used car.
That was the good part!
I'm going to try to buy, either a rebuilt or a good low-mile battery, from a totaled car. I might even consider rebuilding the pack, myself.
I guess, all good things have to come to an end.