My last oil change I used this. I've kept fuel logs since owning the car (30000 miles) the following two fuel fillups were the two highest MPG figures I've seen since owning the car.
It wasn't a MASSIVE difference, but it will pay for itself in 3000 miles.
My average mpg since owning the car (spirited / motorway driving) = 29.4 (124 fills)
Average since starting new job (communting only) = 33.6 (5 fills)
Average since starting new job + nanodrive (commuting and spirited) = 35 mpg (2 fills)
It's still early days