Really an interesting read between this thread and the down jacket thread. My experience is that a 100 weight garment is perfect for me while moving in cold weather. It packs better too. I usually wear a wicking layer with a Polartec 100 jacket as my standard. Was out snowshoeing in low teens (F) this last weekend with this system and kept completely warm. I also go with a Polartec 100 pair of pants. For long stops, a puff layer keeps me warm. I also carry a lightweight Primaloft vest that can be added to the fleece if conditions warrent it, and while skiing I often substitute the vest for the jacket to give my arms some additional movement freedom. Bottom line is that puff layers (down or synthetic) are my "non-aerobic" insulation layer, with light fleece my "aerobic" layer. Now if I were climbing Everest or something, that down climbing suit might be more appropriate, but in stuff just a stone's throw from zero F (either direction) I like my system.