I also have the Conti's on my 330xi and I was not impressed with the snow performance. I have always tried to run snow tires on all my cars.
Currently I'm running the Nokian Hakkapelliitta Rsi on the 330 this year. Luckily I had them put on the car last Monday because this weekend here in Denver we had some sizeable accumulations and the car performed like a charm.
Now I'm not just talking about acceleration were the all-wheel drive comes into play. I'm talking about stopping the damn thing. I was quit impressed with the Rsi tires ability to bite and the bimmer's ABS and Traction Controls to keep her managed.
All in all I would also HIGHLY recommend a dedicated snow tire for the winter season. The Nokian Rsi tires get thumbs up IMO. Another winter tire to consider is the Bridgestone Blizzak WS-50. This uses a multi-cell rubber compound which is also a great winter tire and excels in icy conditions.
Let me know if you have any other questions. Here are some links for info on the tires mentioned.
http://www.nokian.com/passengercars_product_en?product=610499&name=NOKIA N+HAKKAPELIITTA+RSi
http://www.bridgestonetire.com/tireselector/index_bs.asp CHOOSE: Catagory, Winter, WS-50
Cheers,
PR 