I'd say goog to keep you'r feet warm through high 30's. Probably more ideal in low-mid 40's. fit is good. My shoe size is 44.5 specialized s-works, Large fits well in the Narcisista. I also have the Nanoflex and a large was too small -felt like the seems where going to burst at any moment. its hard to compare these to the nano's....these seem to have a boader range of temps (that is you can keep them on when it gets into 60's), but they dont do as well in the wind or rain. They are more comfortable, but dont have that quite as much of an aero feel. Nano's are better in wind and moist. Back zip on naon's sucks and digs into skin. side zip on these is great. the narcissita would be better if it had a wind-proof toe covering.