In reference to the comments on breathability, in my experience Gore's standard Windstopper doesn't breath that well in the scheme of things for high output. The Gore Windstopper Next to Skin in my old Mountain Hardwear Transition tights seems better, but apparently neither are made anymore. The full length zipper of this jacket gives you a lot of control over venting though. The mesh over the back is awesome. I find this more valuable on the down than on the up.
If I'm really working hard, it's not windy, and above 10 degrees I find I need to drop this layer on the uphill. On windy days though, it's nice to have, especially if I'm not working hard. I've mainly used this jacket for training sessions at the resort and for that I love it. I keep it easily accessible in my pack and throw it on quickly at the top and take it off once I transition for the next lap. At 10-20 degrees with a light wind I wear this on 2,000' of descent wearing just a 165 g/m merino layer and Polartec Wind Pro gloves and I don't feel cold at all on the descent.
I'm 5'10" 150# and a 38" chest and the US small is pretty athletically fitted on me, but I can still fit an Atom LT jacket under it if needed but things start to get tight there. Thumb loops are nicely done such that they aren't too much in the way and they don't leak air too much when not being used. The jacket doesn't stretch that much though so trying to reach down and undo a binding is a little difficult if the thumb loops are on. The internal skin pockets are really only good for a race skin as they are very narrow.
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