This is a bunch of first impressions on the Arc'teryx Proton LT (a jacket I've not seen mentioned here) and the short and the long of it is I'm really impressed. Of course, it could be a case of puppy love as I've only used it in ideal conditions: relatively high activity in dry sub-zero cold which is what it's designed for. For one thing, the copy writers were not lying when they said it was breathable. So much so in fact that when I put it on from a cold start with some seriously cold strong wind I wondered how it could keep me warm. I read a review that said it was windproof but whoever wrote that must have tested it in front of the office fan. Having said that, the face fabric does shed quite a bit of wind, nothing like a wind jacket but a heck of a lot more than a fleece. It really did come into its own once I got moving though and shed just enough heat to keep me in the sweet spot except during long ascents in the sun which will had me down to base layer. During short breaks, the rapid cooling down was in fact welcome although I would have thrown a light wind jacket on top if I had spent a long time being static in the wind. The cut is pretty trim which is good for something meant to replace a fleece and makes it easy to wear it under a puffy or a shell. The hood is great and it is one of the only jackets that is actually comfortable when worn fully zipped up but with the hood down. It doesn't stuff into a chest pocket if you like that sort of thing but it's easy enough to stuff it into one of the sleeves to get a manageable compact package in a pack. So far so good but I'll have to try it in the dreaded 0 to 5 degrees wet weather to see if it's a one-trick pony or something more versatile. The best things is that it's only about 30gr heavier than my hoodless Monkeyman fleece.