$683 seems like a lot of
$683 seems like a lot of money for something that may or may not extend your battery life a little. The feedback on PriusChat on this is pretty mixed. How could you prove that it works in the long run ?
Prius HV battery failures are pretty rare, and this seems like FUD being spread about Prius batteries. We just sold our 16yr old Prius with at 208,000 miles for a pretty decent amount, and there's no way we could have gotten $683 extra by showing the buyer we had used the Prolong. Sure our yearly MPG slowly went down from 51 to 48 over the last 5 years, but it still runs. And there's lots of refurbished batteries out there.
Would your rather drop $683 today, or take the 50/50 chance that you might have to spend a grand for a refurbished battery many years down the road?