There are two basic types of winter tires: Severe service winter or ice tires, and European or high-performance winter tires. The former is better on slick ice due to softer rubber, and the latter is better for highway driving. Ice tires will wear out faster and make more noise especially on the highway. They also do not handle as well because of tread flexing (feels a little 'loose' at high speeds).
All winter tires are only useful in winter until the tread is worn to about 6 mm or 7/32" to 8/32" (they usually start at 10 to 12 mm). The 'sipes' which are cut into the tread blocks and the soft rubber compound usually only go down that far, plus the tread becomes too shallow to displace snow effectively. After this point, many manufacturers recommend that you leave them on the car through the summer, and replace them the next fall. Or you can just replace them. Some fast-wearing ice tires like certain Blizzaks, are designed to be installed in the fall, used for 12 months, and replaced every fall. If you don't drive enough miles to wear them out in a year, then that scheme probably isn't worth it for you.
I use Michelin Pilot Alpin PA2, which are the European type, and I like them. I chose to use 215/55R17, which is the same diameter as the stock, but narrower, so they will float around on top the snow less. They wore less than 2 mm in the 5 months they were driven last winter (1/2 city 1/2 hwy), so I should get 3 or 4 winters out of them. Good thing too... they were not cheap.
