To eventual an buyer:
About a month ago I was hunting for a lens like this and when finally when I decided to purchase one, surprise... the lens was acting much worse with the IS on then off.
The lens is still listed on Craig List, so be aware. I have advised the seller to claim the warranty, but it looks he prefer to sell it, so be careful. Just to show that I have more respect then him, I'll not point exactly to the listing.
To the seller:
Just claim the warranty. Think twice before selling a the lemon...