spdrcr: you've never had a Porsche then
It depends on the method used to make them. Cast rotors with drill holes are perfectly fine, drilled rotors after making them are prone to issues like cracking..
That said, you'd have to be doing some real thrashing to crack a good set of BMW rotors. The M3 has them and various other M cars all with much success
If only for the weight saving, these are fantastic. 3.1lb per corner is BIG