It worked like a charm for our crew at Plummer Hut on a climb of Waddington some years ago. Weather was sunny and cool to warmish(~50 degrees) by mid afternoon, but mostly not windy. We set the bag on a platform of exposed rock, not on snow, and started with wet snow. We didn't try it up higher when we were totally on snow at around 10,000', so I can't vouch for how universally applicable the technique is. I've also seen people use the technique in early spring conditions in the Cascades. I think the trick is to insulate the bag from the underlying snow, flatten it out to provide maximum surface area facing the sun, and keep it out of the wind.