First off, the jacket temp ratings are totally relative to each individual. Some people will find this jacket runs too hot for their intended purposes and others will find its not as warm as claimed - I fall into the latter category as I tend to run very cold. This jacket works well for me down to about 40 degrees and as high as about 60 degrees. Below 40 I usually wear one of my hiking shells with a warmer jacket underneath since this jacket doesn't have much room for layering. The dual zipper design is perfect as it allows for adaptability, especially on those days where you start early and end late and the temperature warms up quite a bit. Gore Windstopper isn't as breathable as other softshell fabrics but I don't tend to sweat a ton so this is a minor issue for me. Overall, the jacket fits nice and trim, blocks the wind and some rain, and provides enough warmth for average winter weather in Seattle.
I am 6' and 135 lbs and bought a large (I do have a small torso though with longer limbs). With Castelli, size up as a general rule.