120 mile round trip today I managed 27.3mpg, and I wasn't exactly hanging around either! Mixture of all different types of road, and varied traffic/speed/acceleration levels. Quite impressed actually! On a 2 hour motorway journey the other day I managed 31mpg.
When ragging the car, I saw 19mpg, which again isn't too bad.
But you don't buy an M3 for the mpg, so the above is pretty redundant to be honest!