You have to ride them gently for a bit to begin with so that the studs can set properly, but they grip just fine on ice from day one. You can get by with one on the front for steering, but if it's really icy out you can risk your rear wheel, the one with the oomph, sliding out from under you. But that's usually not a big deal at moderate to low speed.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett