I've got WS-60 Blizzaks on my 135i. They are incredible. I can jump off the line with minimal slip (a light foot is required), corner with confidence, and most importantly, stop on a dime.
I was having a little discussion with a co-worker who drives an '04 Audi A4 avant with all seasons. He claimed that his quattro didn't need winters to start/stop as quickly as mine with winters. I disagreed.
Over lunch, we put together a little test in the parking lot which has a light dusting of snow over ice yesterday. Take off was pretty close...but I embarrassed him when it came to stopping distance - over a car length.
You're right johngenx, "it's all about the tires..."