If I'm skiing a half day or night skiing I do not wear one. On full days I always wear one. Extra gloves and liners, base layers, car keys, cell phone (only used for emergencies), iPod, and Cliff Bars or something similar. I have last year's version on this. The H20 reservoir is the main reason I got around to buying a pack. http://www.backcountry.com/store/CAM0206/CamelBak-Menace-Hydration-Pack.html?id=5aspzXqM
That's pretty cool, kinda what I've been looking for. Price seems a little steep, but nice to have on longer days and to haul a few extra things for the kids around.