When I was driving the 330d I had the chance to try both my summer and winter sets on the same day, on the same snow/ice covered lane. Was able to make some comparison tests, to really see for myself what the differences were.
Where the summer tyres would slip and slide about, no acceleration and braking was just a slide, the winter tyres transformed the car. You realise how "on the edge" you really are on summer tyres, and how much safety the winter tyres add, even to smooth ice.
Acceleration and braking were in a different league, almost as if there wasn't any issue with snow and ice under the car. When I flipped it through a 90-degree junction, the summer's had the car swinging out, DSC light flashing as the car tried for control again. On the winter tyres it was very hard to even get the rear to break away, let alone see flashing lights.
A few weeks ago I had my 5-series on the same lane on a surface (much as the picture of it outside my home), and tried the same acceleration/braking, plus the rear swing out, again no drama, very impressed. Also driving through deep slush, (one of the worst conditions for a BMW, in my experience) very impressive performance.
I'm using OEM 225/55 R17 Dunlop SP WinterSport 3D run-flats.
HighlandPete