My guides on Rainier all had one. I was skeptical since the material seemed so thin and flimsy, yet they all swore by it. I gave it a shot and... wow.
Breathes like a t-shirt but blocks *most* wind. Seriously, it seems impossible given how thin and light this thing is but IT WORKS. Also surprising how burly the material is, because nothing seems to hurt it (rocks, trees, and incidental contact with pointy things like pons and tools). And it's super light and packs tiny into its own pocket, and pops out not wrinkly. OR has some remarkable fabric here.
It will wet through in decent rain or if you're leaning against a seep, but other than that, snow and light sprinkles will bead off.
The hood fits great when worn without a hat for running in windy, wet weather, and it seems like it will be too snug for a helmet - wrong! It's so stretchy it fits perfectly over a helmet, fully zipped, without being too snug against the chin.
The fit is perfect - athletic builds (read: have a chest and arms) will appreciate the stretch. It moves with me perfectly and the arms are long enough that they don't pull away and the jacket doesn't ride up above your waist when reaching. The hem cinches too, which also helps keep it in place. I love that there are no fiddly wrist adjustments - they're snug but not tight, just perfect.
The only conceivable downside I can think of is that the hand pockets get covered by a waist belt or harness. But it has a chest pocket, and my legwear or pack is where I put most stuff anyway so it doesn't bother me.
It is useful for seemingly every scenario, and I bring it on every hike, climb, or camping trip. Fits great, looks great, is great. I simply can't say enough good things about the Ferrosi. Get one - when perfect gear comes along, you have to snag it.