I was excited to try the new Proton FL jacket. It fit just slightly slimmer than my Atom LT in the same size. I could fit a baselayer under it with room for a medium fleece, and even a heavier weight fleece worked, making the jacket fitted, since there is a little stretch to the Proton fabric. I liked the direct insulation, and it would make it a good jacket to wear just over a short sleeve top, since the lining feels nice on bare arms. The hood is a little different in that it only has insulation part way up. I'm sure that's for breathability. My only concern was that I could see tearing right through it if I hung the jacket by the hood on a hook. Of course that could be avoided. I would have kept the jacket, if it wasn't for the fact that when zipped up all the way with the hood up, the front came clear up over my mouth. That's a good feature in a rain jacket, but it's not something I want in a lightweight jacket/mid-layer like this. With the hood down, I could push it under my chin, but then it ballooned out and the zipper was uncomfortable against my neck. You can actually see that it fits the same way on the model in the photos on the Arcteryx website. To some people the tall collar is probably an advantage, and it may not fit other people the same way as me and the Arcteryx model , so I do recommend the jacket if you're considering it.