I live in Ontario and ran my 07 335i last year with 18" winters. No issue at all. I drive about 15-20km a day for commuting.
I used WinterClaw tires as I've used them on my last 3 vehicles. They're ice rated and cheap.
My only advice, keep the traction on unless you're looking to have some fun...I was surprised how well the car behaved. With traction off it was ridiculous, but what do you expect when you're spinning one wheel.