Just got an MK3 and took it out for its first dive today. I have to say that I'm impressed and in the spirit to providing helpful information I'll be posting up performance reviews for my dives this weekend and the next.
Today's dive was 74min in 42ºF (5.6ºC - averaged from 4 computers). Was wearing the MK3 with a t-shirt and long john pants underneath (no top though). The dive was a training dive so lots of hovering and doing S, valve and ascent drills. Including surface swimming and chatting on the surface the total in water time was at least 100min. Started out feeling almost too warm, by the time we got out of the water I was warm enough that I would have been happy to continue diving for a while. I could definitely feel that I was losing some heat but I wasn't even to the point of feeling the chill creep in. By comparison, after 100min in the water with the MK3 I was warmer than I usually was with my BARE HI-Loft after about 30. Just bought a polartec shirt to wear under the mk3, partly because I want to see how warm I'll be with the extra layer, but mostly because I want to keep my mk3 smelling as fresh as possible for as long as possible. Next set of dives is tomorrow, I'll let you know how it holds up.
Edit: Forgot to mention. I didn't have any mobility issues with the mk3 when it came to reaching valves, clipping and unclipping various odds and ends, donnig/doffing gear etc. but it is a bit bulkier than I'm used to and my suit is a little skinnier than I'd like it to be, so squeezing into my drysuit was, if anything else, entertaining for those who were watching.