I buy from discount. The warranty is legit.. But as it clearly states it is for 3 years. Not a lifetime. Since I have never had an ST tire last 2 years, it is a moot point.
Since I have been using Michelin XPS RIBS since 2006, I don't buy the warranty on them. I replace them at the 5 year mark. Never had a failure and they take LOTS of abuse. High speed, curbings, washboard roads, nothing fazes them. Since they have lots of tread left and look good, I sell them on craigs list. They sell really fast @ 150.00 for four. Get a nice chunk of change to put towards the new set.