Loads have tried it but it is a complete waste of money! the MPG is the first thing i noticed when i got my IS, there's no way of getting round it, its just part and parcel of owning the car.
If you try the super unleaded you have to wait for a couple of fill ups as the system would need to be flushed of the standard stuff.
In my experience though, the MPG gain evens out when compared with the cost of a full tank, only the super is better on the engine.
You could always get an LPG conversion, but it's a pricey install and it'll take roughly a year for it to pay for itself but you'll get 3 times more miles out of the £60 you currently spend on filling your car.