My experience somewhat parallels Scot's. Last time i used it, was early spring at SNP. I was wearing a thin, linen button up shirt, with a windshirt over it (no hat). The low got down to around freezing (a little above i think), and i was more or less comfortable just sitting around when i put the Hadron over these, but occasionally thought maybe i should put another layer on underneath. But i should mention that linen is a very cool fabric to begin with, and provides very little insulation.
If i had had a mid weight wool baselayer or some kind of thermal mid layer on, i would have been completely toasty.
It's definitely not for extreme cold, but for colder situations i have combined it with a warmer baselayer, a thermal mid layer (not too thick though), a warm hat/gloves, and a lightweight down vest.
It's efficient at using what down it does have, because of the hood and the anorak styling. Those are big pluses in my book.