This is my go to layer for skiing on any day below 40 degrees F. I can understand the criticisms of some of the other reviewers here, but because of the way I use this jacket (which might differ a bit from them) it works perfectly for me and I give it 5 stars.
I wear this under a shell jacket and it has finally solved my layering problems for the first layer. My problem when skiing is I tend to be either too hot or too cold, and sometimes both if that makes any sense. My base layer also needs to be multipurpose so that I have options when adding another layer. So I need a layer that gives moderate insulation but breathes very well. This is where the Atom really shines. I can have it zipped open when I'm too warm, and then it feels like nothing is on me. The side panels are fantastic. When it's cooler and the Atom is zipped, it gives great insulation and is the only layer I need down to about 20 degrees F (depending on wind and sun conditions). But the real surprise is what happens when it's colder. My second layer is a Patagonia Down Shirt. When it's not too cold, the down shirt is over the Atom jacket. This way the combo breathes more and gives less insulation. When it gets even colder, I reverse the order and wear the down shirt under the Atom. This way I get more insulation and it breathes a bit less. With all the possible zipper adjustments of both layers, it's easy to handle anything down to about 5 degrees F (again depending on wind and sun). When it gets mega cold I add a Patagonia down sweater. Due to its light weight and packability, it's easy for me to stuff the Atom in my backpack, even with a lot of other gear there. Another nice feature is the pockets, which work quite well for me. When the outside pockets are zipped open, one gets extra breathability. I use the inside, chest level pocket to keep my cell phone from freezing up. If it gets to minus ten F with heavy winds, I can move my cell phone and put a body warmer in the chest level pocket.
Where I differ from other reviewers is perhaps how I wear my Atom jacket. It is true that wind can blow in the sleeves, but this never happens to me because the Atom is under my shell. It is also true that wind will blow up from the bottom of the Atom because there is no waist cinch. But if it is warm then this is an advantage, and when it gets colder I always tuck my layers into my ski pants, hence no wind problem. (My core tends to easily get very cold.) Yes, it looks lame tucked in, but hey, it's underneath my shell so nobody sees it. Perhaps Arcteryx might want to reconsider the length of the Atom, which I think is a couple of inches too short. But because of the way I use it, it's not a problem for me.