Friends with Prius':
2001 Gen 1: bought new 210,000 miles, no problems - sold for $6,500 a year ago - still running fine (another friend bought it).
2004 Gen 2: bought new 45k - no problems. (rides a bicycle a lot)
2004 Gen 2: bought new 320,000 miles no problems.
2004 Gen 2; bought new 130,000 or so miles - no problems
2005 Gen 2; bought new 218,000 miles (last I heard) - no problems.
I haven't heard of these people, or anyone, needing to buy a battery pack (yet). I don't know the Saturn/GM battery life span though.