I was surprised at the difference in MPG I got when switching from regular to premium + the slight performance enhancement is worthwhile.
All of the 5 series have a knock control sensor which allows you to use pretty much what ever fuel you like and it will adjust to it. Premium is obviously the best choice (otherwise it wouldn't be called premium) and all the performance figures you will see from your BMW manual (mpg, mph 0-60 etc.) are taken using the premium fuel grade in the country the car is sold in - so to achieve these figs. you have to use premium.
It certainly shouldn't have bothered the engine in anyway but you're running a beamer not a honda - treat her nicely and buy her the best.