I don't have an M3 but someday I will. In the meantime I have a 328 with a manual. I let my gf take my car if she needs it whenever because she drives a Mini Cooper hardtop with a manual. Granted its an easier manual to drive, but the Mini is also a lot lower to the ground than my car.
So if she needs my car I have one rule... If you're going to get in an accident then total it. Every time She takes it I remind her to be careful (she's a really aggressive driver) and that the car is replaceable but human an pet life isn't.