I do.
Since I really can't be bothered to a) fit my fuel computer or b) sit and work out MPG. I just know £20, at the moment, will see me 150 miles down the road. It's actually quite a nice way to directly compare economy. £10 of LPG, after all, tends to get one a lot further.
It also keeps you informed of the changing cost of motoring. Gone are the days of my 10p mile.