I just purchased the USAA extended warranty for 60 months or 75000 miles. Paid 2K. I never buy the extended warranty, but after I had some issues with my out of warranty X5, I realized it's worth it for the peace of mind. Plus, BMW repairs aren't cheap.
I figure the M3 being as high strung as it is, might get really expensive fast, especially if you have engine or transmission problems.