This is the only time I've purchased a Carhartt product that -- in my own experience -- ran just a bit small as a 3X, so I purchased a 4X and it was perfectly fine. I'm 6'3" and about 340 -- I let myself put on too many pounds so I've got the gut thing for now, and I found the 3X to "ride up" in front a bit more than I wanted. I'm keeping both sizes and can use the 3X once I'm back in shape.
In terms of "super cold" as an insulating layer, I'd say as a cotton garment it's probably better suited for less-than-extreme conditions and less-strenuous physical activities. Some other manufacturers make synthetic baselayers that are better suited for strenuous activity in extreme conditions, and as a result make layering simpler. This Carhartt actually breathes very well and I like it as a base under a flannel shirt, jacket, etc but it's TERRIFIC as a standalone piece for temps in the 50s - 60s. If they made it with a chest pocket I'd buy that too!
One other thing I like is it doesn't ride-up on the backside - it's nicely tapered for an active fit.
Clothing is like food -- entirely subjective -- so don't rely on reviews for fit, comfort, use. Buy one and if it fits and works for your application keep it. I like this, but I'm in California and don't have to deal with extreme cold. I happen to like it precisely because it's NOT an extreme-cold garment, and is useful and practical for multiple uses.