I've been testing out the 2008 fall gamma mx for the last couple months in Canada/Chicago weather. I like to wait at least a season before making a review, but the recent negative reviews are bugging me.
This jacket isn't supposed to be insulating. That's what you wear an insulating layer for. Buy a down jacket or insulated soft shell if you want a warm shell, at the cost of versatility. This jacket does exactly what it's meant to. The pockets rock, the attention to detail is everywhere, it is comfy, stretchy, bombproof, very wind resistant, water resistant, super breathable, and warmer than a 100 weight fleece on its own.
It is also roomy enough to layer things underneath. At 6" 185 pounds, wearing a large, I can fit a base layer and MH monkey man fleece comfortably underneath. With a 100 weight fleece and this I can go down to the 40s. With the monkey man I haven't hit a lower level yet.
Overall it's an awesome shell. Just keep in mind it is not insulating (most softshells are not, the gamma mx is no different), and you'll need another layer if it gets cold.