"Couldn't believe how light and perfectly fitted it was when I first pulled it out of the package."
You know if its Mammut its quality. The medium fits me great, I'm about a 39 chest. No chills when I'm up high in the early morning, though, I haven't taken it below 30 deg yet- that will happen in another two weeks. I love how light and soft it is- hardly know you've got it on apart from the fact that you feel like an oven in about 5 minutes.
I've used this jacket under my hardshell while ice climbing and skiing and it works very well. Very insulating for the weight and the cut is quite athletic. So if you have a bit of a beer gut I would go for something more boxy like a Ghost Whisperer or Western Mountaineering Quickflash. Breathability is great and it stuffs down very small. I'm 6'0" 175lbs and a large fits perfect.