ZMOOS wrote an answer about Arc'teryx Alpha SL Pullover - Men's on November 4, 2010
I would say that there are better options out there. This is an amazing jacket, but if you're going to be beating on it falling on snow and ice and possible encounters with trees and things I would recommend something more durable. It is super light and packs down to be tiny, but in accomplishing those two things they sacrifice a bit of durability and breathability with just the GORE-TEX Packlite. In addition, to save weight they only put the zip halfway down, which I imagine would get annoying pretty quick if you had to keep taking it on and off.