Winter tires are a must have for Calgary. They slip like 1/10th as much as an all season tire, and, I bet, will probably save the average driver from one fender bender during the life of the tire.
Good link for winter tire reviews:
http://www.canadiandriver.com/winter/tires/tires.htm
I took a long look at the Nokians also, but ended up buying Michelin X-ice winter tires. Mainly because I still had a brand new set of all season tires to use in the summer, and the Nokians cost way more than the Michelin. If you are looking at buying two sets of new tires and extra rims, the Nokians are probably a good buy, for a true year-round tire. I don't think they are worth it if you already have good summer tires with lots of tread. Better to get winter tires, and buy an all-weather tire down the road when you wear out the first tires.
Also, I believe Costco (and other?) tire centers have a $100 rebate on right now for Michelin tires.