I purchased this jacket with the intention of getting a softshell that's more waterproof than your typical Polartech Powershield/Schoeller based garments but more breathable than a gore-tex shell. So I will be comparing the general warmth/feature-set/breathability/waterproofness to an Arc'Teryx Gamma AR. Now, while generally my experiences with Mountain Hardware gear has been generally described as "meh," the G50 has some great features that for once put it on the same league as the Arc'Teryx softshells.
Fit: It sizes small, although not as small as the skintight shells like the Arc'Teryx Gamma MX's. I generally wear a medium in Marmot, Arc'Teryx, and Helly Hansen and for this jacket I needed a large to get a nice tight (but not constricting) perfectly sized fit. I'm 5'7 150lbs, for the record. When I'm wearing this with a harness, the length is ideal since it doesn't interfere with its placement (or get tangled with my cams).
Warmth: On its own, it appears to be a laminated material so it's pretty thin backed with a minimal fleece. I'd probably wear it alone comfortably from 40 to 60 but definitely not lower. However, this shell doesn't really stand out for standalone wear...
I layered with a Patagonia Capilene Expedition weight crew for the baselayer, then the Arc'Teryx Atom LT for an insanely warm synthetic midlayer, and tossed the G50 on top as a nice shell. I tried out the combo at night during a chicago winter at 10F with 20'ish MPH winds next to the lake. Standing still I couldn't feel a thing.
Breathability: Running, the combination of layers breathed quite well, although with intense work I did need to open the hand pockets a bit for ventilation. Generally it kept me dry.
Waterproofness: During a rainstorm at around 32F I went for a run with a synthetic Helly Hansen baselayer and the G50 as an outer. It shed water for around an hour, but the seams definitely did begin leaking a bit after that. The synchro jacket has taped seams, but of course that sacrifices some breathability. I personally can live with a bit of leaking, but downright amazing performance for a softshell. If you want pure waterproofness, (aka an unexpected rainstorm during a climb) then you'll probably want a really lightweight gore-tex rain shell to throw on top.
Unlike Arc'Teryx gear, you don't have to pay an arm and a leg for this. For $170 (I actually got this for $120) you're getting a really solid piece of gear.