Good luck with that. Anecdotal evidence is better on this forum that that will ever be.
Here's an example of what I learned about S5's from lurking on the audi forums: Somewhere between 50k and 100k (right out of warranty, but probably closer to 50k) The timing chain tensioners will wear out and require replacement for the "lifetime" timing chain, which is of course now packed into the rear of the engine. It's about $5 of plastic that needs to be replaced, and for that the total bill is along the lines of $7000, since the engine needs to be pulled.
One repair like that on the M3 would make the extended warranty worth the cost.
Fortunately I'm 4 years out from the first M3, so I'll get some data on you guys before my warranty runs out