Just took the Rab Infinity out for its first spin: several days of minus 4 degree temps in Bryce Canyon, Utah. This jacket was outstanding! Tired of being cold wearing my Patagonia down sweater in sub 30 temps, I purchased this jacket based on backcountry.com reviews and online help. I got exactly what I was looking for: outstanding warmth in low temps,even during long lunch breaks.
The fit is perfect. I am 5'7" and weigh 120#. I bought the size small, which fit perfectly over layers (wool tee, long-sleeve shirt, beanie and even a fleece sweater, but that was overkill). It also layered nicely under my waterproof jacket.
Two pluses (beyond the incomparable warmth) are the handwarmer pockets and the hood. Wearing only liner gloves for picture-taking dexterity, the pockets kept my hands plenty warm. The hood fit easily over my beanie, and the elasticized edge is a nice tough, keeping the fit very comfortable and helping to hold in the heat. Also, the soft fabric near the chin was a very pleasant discovery.
Only gripe -- the color and odd sheen. I have the Shark color, shiny and not flattering by any stretch of the imagination. But I bought the jacket for warmth, and it has delivered in spades, so I would highly recommend it for anyone whose priority is to stay warm in bitterly cold temperatures.