In California there is a 10 year warranty on the battery, most other states its 8 yrs. most of the Prius batteries have been lasting at least 10+ years before replacement. In my case with my wifes car, she did most of her driving between home and work, home and grocery store, very rarely taking it out of town. Car was not garaged and in the high desert, temps get to 115 degrees in the summer a lot. wich is bad for any battery. Did my battery fail prematurely?.... probably not, it was definitely stressed in a harsh environment compared to a lot of other vehicles though. Other than cosmetic issues (i.e. road rash, bad parking, etc.) It has had almost no issues, and runs just as good as when we first bought it (With a healthy govt subsidy I have to admit). Would I buy another one?.... Not really as it doesn't meet my needs here, and I don't know how it performs in snow, it is not AWD. The price of the battery has gone down from when we first bought it, it was originally around $4K to replace the battery, its now down to around $3k
For some uses its a good car, gets good mileage and is more confortable than others. For others, not so great. Is it cost effective/economical?.... depends on how you drive and conditions that you drive.