In reply to Bluebird:
I have a Compressor, use it as a belay jacket in winter. It would be too warm/bulky as a mid layer, under a shell in my opinion, then again I sized it to go over everything.
I have an original Prism, definitely not as warm, perfect as a mid layer, similar weight and pack size. I have also been given a Nano Puff Vest, fantastic warmth for very, very little weight.
Stuart