Yeah, I drove an M5 a couple of weeks ago and, even thought it's dificult to tell on a test drive (with someone sat next to you) I felt it only did really shift once you'd got the revs higher. BUT, it did REALLY shift. Problem is, this is when the mpg drops like a stone (less than 10mpg), however even when going gently you only get 15mpg-ish. Motorway is morelike 18-19mpg. My conclusion has been don't buy an M5 unless you're happy with sub-10mpg as that is invariably what you'll end up getting if you drive it 'properly'.
That said, it's still damn tempting....