I'm giving this item three stars but that is more a reflection of my misunderstanding of what the Accelero's intended function is. I bought the Mercury Accelero as an insulating, sunlight reflecting, glacier walking, layer to wear on my trip to Rainier this June. It is an insulating layer only in the sense that any layer offers some degree of insulation.
It is very thin (but rather stiff) and more akin to a wind shirt in my opinion and is best suited as an external layer for cool (not cold) morning runs. It is cut a little large and you can't tuck it in because of a little credit card size pocket located right at the waist line. Nor can you can't cinch the bottom hem tight. Because of these reasons I found that cool breezes kept blowing up my backside freezing my arse off during times of light activity.
It is beautifully made however. I just haven't figured out how to use it other than as a high dollar wind breaker.