Just stuck my snow tyres on the X5 for the first time and what an incredible difference. Accelerating and braking had so much more traction it was like driving on loose gravel and not snow. I have read that they also very good in wet weather so will leave them on for now, another bonus is that the ride is nicer than with 20" run flats that come as standard.
The tyres were excellent value too, I paid £570 including fitting (vredenstien) for 2 x 255 19s and 2x 285 19s, the spare m sport oem 19" rims cost me £600 off ebay so all in all excellent value.
Im driving to Austria next year so the recent snow has given me an excellent excuse to test them out.