Just from personal experience on the small hills I encounter I actually saw a decrease in mileage from this. My guess is that when you turn the key off and back on you reset the ECU and the car goes back into open loop. So unless you have some decent hills/long coasting it might not even be worth it.
Aside from the unknown effects of having the igniton on without running the engine - running the battery down at somewhere around 10-15 amps or more - by simply coasting in neutral with the engine running yesterday with typical stop signs and lights and a short engine off stop talking to a friend then bump starting out of his driveway again I drove about 16 miles at 56.2mpg in my xB. Do you really need any higher mileage than that to be happy?