The versatility of this jacket cannot be overstated. It is warmer than it's weight would imply, resists elements better than you'd expect and more comfortable than its performance would imply. If I am wearing a t-shirt, thin baselayer and the Atom LT, I am pretty comfortable down to the low 30's Fahrenheit. Not bad at all for the weight of the jacket. On the other end of things, I can wear it partially unzipped up to about 55 degrees Fahrenheit. It blocks wind rather well, but still breaths due to the fleece side panels. The zipper is buttery smooth, the waist hem works well and the stretch panel wrists are really the best I have ever encountered on a jacket. I do have to layer with a moderately insulted vest under this to get by in the Idaho winters when it gets into the 20's, but, by doing that, I have avoided having to buy a full-on winter coat (eventually I will, but I love the light insulation pieces for the versatility). I can wear this all day in comfort during the cold months (unzipped inside, zipped outside).
There are two things that I would personally change about the jacket:
First, I would prefer the hood to be of normal size, not an oversized hood that fits over a helmet. While I totally get why Arc'teryx designs most of their jackets this way, I personally do not like it, as I use it for daily life, hiking, ect. I do not do any outdoor activities that require a helmet, so the hood is too roomy at the top of the head (even with the hood cinch, as it cinches more around the lower head and not so much the whole head).
Second, I prefer the Arc'teryx logo with the bird only. I love repping Arc, but I really like the more streamlined look of the bird only, not the bird AND the name "Arc'teryx". It looks a little busy.
These are fairly trivial things (though I really wish the hood was sleeker, I would consider replacing it with another one if they ever release an Atom with a more standard sized hood).
All and all, excellent product, delivers on everything it claims to be and worth the money.