In reply to BnB:
I have the older Prism (25gram fill), not warm enough IMO. It's a great midlayer under a shell, thin, slick, not as a stand alone piece to stay warm in.
Usually if somethings warm enough to belay, it's a pain to hang off your harness and I don't like climbing hard routes with a sack, which is the kinda routes you really NEED one for!!!!
Something is better than nothing though.
Stuart