I used Conti Extreme Winter Contacts on my last car (A4 Quattro) and was very pleased. I switched from some Yokohama all-seasons to the COntis on my winter set of wheels, and the difference was night and day. Unfortunately, they aren't offered in a 18" size. :-(
For anyone buying 17" wheels, I recommend them. They were quiet (I used them for two winters), especially for a dedicated winter tire, and weren't really mushy (the size I used was a 215/50-17 on all four corners).
EDIT: Apparently there is a ContiICEContact to be offered in 18s. I'll check them out. They can be studded.
2008 335i sedan | black/black | Sport | Premium