I bought a downhill ski helmet a few years ago for my winter riding and LOVE it. It's a Giro, I forget which model. I find it's too hot for anything warmer than -5C. I don't need any more layers until about -25C and then I have a balaclava that I wear from -25C to -45C.
It probably depends on the weather in your area. Here on the Canadian prairies it's been perfect.