>Has anyone tried BeeMax mediums as brood chambers in an 'all medium' hive?
Yes.
> If so, how did you like them and how did they overwinter?
I was trying to get nucs through so I only had one box of bees in them and they didn't do that well. A guy here locally has all medium beemax hives and they do awsome for him. I overwinter in all mediums all the time. The bees oeverwinter just as well and probably slightly better in mediums than in deeps as they get stuck on one side or the over of a comb less often with better communication in that gap between the boxes.