In reply to ARussell:
I have a ME Compressor and a Haglofs Barrier Zone Goody.
The Haglofs one is fantasticly warm, bulky, heavy, needs to be carried in a 10L pack (Decathlon £1.99). As a result, is carried by second, hence once you lead, need to wait for second to arrive at belay. During this time you had started to cool down, losing valuable body heat generated whilst climbing.
The much lighter Compressor, packs small, can be hung off harness with belay mitts, donned immediately you've stopped leading, insulating your core from heat loss!
What is the most thermally efficient one to use??
Stuart