I got this primarily for fall/winter trail running, and also to have as a windbreaker that packs down to the size of 2 pairs of undies (though, for climbing I would use something already kind of trashed b/c of abraison). I am 5'5" and 145 and I am happy with the medium. Since I never go on a run with anything thicker than a patagonia R1 over a silkweight layer, I did not buy it to be 'roomy' (though I could fit a medium fleece under it easily). I like the longer length in the the back. The sleeves were a good length- tall gals beware. I do not feel like I need to be gentle on the fabric- it is surprisingly strong for the weight. It breathes pretty well, but when running I am going to get sweaty no matter what, even today when it was 30 degrees and snowing sideways. I had to use the hood in the blowing snow, and I am glad I had it. The cinches on it aren't as easy to grip as the bigger ones on my patagonia rain shell, but I suppose their size is a weight-saving idea. The red color is a lighter red, and I like it even though most of my wardrobe is green and brown and black. And the jacket has a slimming effect, which is always nice.

I agree with KatieSue, I think the Lite-Speed is a good fit, not too tight. I'm 5'11" and 135 lbs. I bought the medium, and am glad I did. It fits over a light fleece with no problem. If you want it to go over a puffy down, def. size up. The cut is great (like the athletic fit of Arc'Teryx), it's super-light, and is the perfect small size to toss in a day pack or take along backpacking in case the weather turns (it stuffs down to the size of a large orange.) I wore it in a downpour recently, and the arms got damp through to the skin pretty quickly, but the shoulders, which are double-layered, held out longer. When the rain tapered off, it dried just as quickly despite the high humidity (I was in the South). My only complaint is I wish the arms were a bit longer. The elastic sits right at my wrist, and I'd like another inch or two. But then again, I am tall, so if you're of a more "normal" height, it should work fine for you. Overall, I highly recommend this jacket!

I got this jacket for climbing since it's so light and packs down small. After a few climbs when I had a hard time enjoying the view from the top due to a cold wind picking up after I was a few pitches up, I wanted something I could attach to the back of my harness for the top but not get in my way as I went up. I love this thing. I didn't think it ran small. I ordered a Large at first but exchanged it for Medium and it fit great, (I'm normally an 8). It's easy to stuff in the sack but doesn't come out looking wrinkled and gross. It's about the size of a softball. The chest pocket is really roomy. The hood packs away really easy. It's incredibly light and breaths well. I wore it around the block in a light rain and it kept me dry too. Not sure about a heavy rain, but it zips up snug and has elastic wrists. I'm pretty broad shouldered and square shaped and it makes me look feminine...but that might just be the purple color. So far I love it.