The Bottom Line: The Arc'teryx Psiphon FL is a pricey but exceptionally useful layering piece for those who run hot but need weather protection. It's my go-to jacket for skinning, hiking on cool, blustery days, classic Nordic skiing, mountaineering, and multi-pitch climbing. It is the Baby Bear of jackets, always just right.
I never would have bought the Arc'teryx Psiphon FL. While a neat looking piece, I had a Squamish Windshirt that seemingly did the same thing, and I didn't need any more lightweight weather resistant layers. But man, am I glad I have it now. It's a bit more functional than a simple windshirt. That extra bit of stretch and material on the arms gives it more durability when jamming hands into cracks on multi-pitches or blocking squalls while ascending up north-facing routes in the summer. I often leave it on when it gets too hot because it does such a good job blocking the sun. And it's so breathable that I can sweat up a storm and still be comfortable.
The weather resistance is superb, especially considering the lightweight material. It blocks the wind, and it's more than enough for light rain. Even damp snow it does great for a while. I've never had any troubles with it as a top layer, no matter how hard it is snowing past -4 degrees Celsius. It is definitely not going to hold up in moderate rain, but it's a softshell.
The HemLock system is just simply the best. Not having to worry about my jacket rising up or bunching up under a pack strap or harness is great. Most of the time, being the skinny guy that I am, this is a constant frustration. But no longer! It also has a helmet-compatible hood, which is a good call.
The pockets aren't anything special. The hand pockets are nice about camp, but they sit a bit low to be used when wearing a harness. The chest pocket is decent for a topo print out and maybe a few snacks. But a map will have to be folded and squished in.
This is really the jacket that I grab for nearly everything. If ever I leave it at home, I always regret it. It's just one of those things that works perfect nearly all the time.