I've only worn this jacket for one day, so I can't comment too much yet on it's warmth (though it was a brisk windy 20 degrees and seemed warm with just a thin shirt underneath). As for sizing: I'm 5'7, 110 lbs. with a very long torso for my height and bought the XS, as I typically do with Columbia clothing. I'll admit to being a small-boned skinny girl, and I was surprised the XS in this jacket is almost too tight. It's wearable and looks flattering (even my conservative dad said it didn't look too tight), but it fits me like a second skin and there's no way I can wear a fleece or thick sweater underneath because it fits so closely in the waist. Arm length and body length are great - it comes just to the bottom of my bum. The black has a nice matte finish. I'm not a big fan of the appearance of the silver Omni-Heat lining, but if it actually works as it's designed, then it's worth it.
I'd say if you are between sizes, size up for sure - this one runs tight in the waist, and since there is a cinch cord at the waist, the next size up should still be flattering.