5'8 144lbs, athletic build, medium. Medium fits great for me. A small would be too snug and a large would've been too loose. It resists wind great (pacific northwest), feels great against my skin, keeps me warm, abrasion resistant (caught the corner of a brick wall = no mark), strong zippers, comfortable cuffs, and very stylish (I got the black). Nice all-around, never have to take off even in your sleep jacket. Threw all my other jackets into the donation box (except my rain jacket, I live in the PNW).
So I am not an adventurer, I'm not climbing the Andes anytime soon. However, I live in a colder climate where you'll see -55* windchill, and usually I wear a pullover fleece and an 800 down jacket to get the paper. However, with the isotherm hoody, I throw it on over a long sleeve shirt, and I'm good to go. I bought it for warmth, but I really purchased it because it zips into the hood as apposed to having an independent hood. I have a climbing jacket from Marmot that changed my life when I found jackets that zip into the hood. Pros:-Warm-Light weight-Zips into hood-Comfortable pocketsCons:-When you put on the jacket, the inner lining can come out of the cuffs a bit... Not a big deal just annoying.
Was searching for a warm layer for hiking/backpacking as well as an everyday mild winter jacket. I knew that was a tall order but I wanted maximum bang for the buck. I did some research and narrowed my search down to this jacket or the Atom LT or Nuclei from Arcteryx. They were all similar in price, features and quality. I decided on the Isotherm due to the use of Polartec Alpha as the insulating material. According to Polartec and Marmot, the new Alpha material is super warm for its weight and allows for excellent breathability. This is a big advantage over other synthetics. When your activity slows down you won't overheat and need to remove the jacket. I was skeptical because the material is so light. I tried it out wearing only a t-shirt and the jacket in -15 degree C weather and I was plenty warm. Then into a warm car and didn't overheat. Looks and fits great, weighs nothing and packs to softball size. It will be my go-to shoulder season jacket. Definitely a winner!