This past saturday (22 Sept 2012), I completed the Lake Michigan 1000. 1020 miles in approx 20 hours. (All documented and submitted)
I had fairly thin gloves on and relied heavily on my heated grips to keep my hands warm.
Over the course of 20 hours I, basically, slow-cooked my hands. By the time I realized what they were doing to my hands, it was too late.
I've got huge blisters all over my hands and they are very painful. It's been 3 days and I'm guessing it will be at least another week until my hands are somewhat back to normal.
(Note, I mainly ran the heated grips on the low setting...occasionally switching them off and also occasionally running them on high.)