Oh, it's low-impact all right, since you spend all your time sliding your feet inches above the ground. You can't have totally cruddy knees, though, as the chance of twisting a knee in a fall is there (since you're essentially wearing a huge lever on your foot). But that's not until you get up to high speeds and/or start getting reckless, so nothing to worry about really. We don't have tracks rated by difficulty so I can't speak for that, but in general falls are more like slides, you sit down and skid along until you stop. On an easy, flat track you'd just sit down, no sliding.
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