I've been running Conti DWS tires for 3+ years on my 135i because of their cost, tread life, and reputation as being a good snow performer. I'm in Illinois and we occasionally get heavy snow and very cold temperatures. I've gone from using them year round, to only using them for 3 seasons a year for the reasons below.
When I first bought the tires (225 up front and 255 at the rear) and they had deep tread, they did fairly well in the snow. Still didn't have the amount of grip that I would ideally want, but hey, it was an all season tire I didn't get stranded anywhere. Different story when year two rolled around and I had less tread. Quite a few white knuckle situations with grip which made me decide I should try the dedicated winter tire route.
Now that I've had winter tires, I'll never drive in winter without them. Even though the DWS (when new) are decent in snow, they don't compare to snow tires, and if your DWSs get below 7/32" you're not going to enjoy it. The tread pattern, compound and tire size (215s vs 255s) make a huge difference. I actually look forward to snow simply to test out the tires now. Ya can't go wrong with buying a good set of winter tires, definitely a smart move.