I'm getting 17 mpg avg according to what the car is telling me. The problem is it's a high performance car (which we usually ignore mpg) and a commuter car (expect reasonable mpg) at the same time so when driving normally, you may feel conflicted as to why a car like this is getting such horrible mpg. When I was in my Evo, or GT3, they were such hard hitting cars, that I never counted mpg becuase I had my fun and got out of the car. Throwing $150-200 for fuel for a trackday (1 day) was fine. When sitting in the M5 for a prolonged period of time and not getting on it and getting 8-10 mpg city just driving does get painful regardless of how much money anyone makes.
My other car happens to be a prius, and what hybrid ecar haters don't realize is, yeah, it's gonna take you 10 years to make up the dollars in fuel to offset the cost of the car but what the real gain is you're saving TIME. People like us who buy M5's know that time is precious. Taking 10-15 out of your commute once a week to get out of your car in the winter and pump $80 is the real waste.