Perfect for trail running - it's nearly as light as the Patagonia Houdini, but has the benefit of actually being waterproof. It's my go-to jacket for backpacking and fastpacking/ultras because of how insanely light and packable it is. I also always keep it my pack while bike commuting around soggy/steamy Atlanta. Even in pouring rain while riding a bike it has kept my top bone-dry and it breathes great. Basically the only thing I don't use it for is mountaineering, where I use a thicker, more durable shell. I'm 5'7" and 145lbs. The medium fits perfectly, although potentially a bit short on my longer-than-average torso. But that's negligible. Highly recommend this one!