Regarding mileage, I'm seeing ~17.5 mpg on average. Probably 60% highway, but I have a major lead foot so this is definitely not the best mileage possible.
My wife is getting ~18 on her X6 35i, with nearly 100% city driving. She's definitely not as aggressive of a driver (I'm always in Sport more, she never is).
I am shocked how close the 50i actually is to the 35i regarding mileage.