In reply to maxsmith:
Montane Icarus is quite slim fitting and has long arms for the size - which, in my experience Arcteyx generally doesn't. I'm not knocking Arcteryx at all, just that I've found the arms could generally do with being longer - for me anyway.
The Icarus is filled with Primaloft Thermoplume, which does seem to have quite a bit of loft - more than say Primaloft Gold. I'm quite impressed with it so far, though I've only had one for a couple of months.