I hadn't thought about bar mitts, but got a pair for Xmas a couple years ago. The ones I got are for upright bars, and they're great. I usually put them on at the point in January when it looks like temps are going to head below 0 F for a while. Normally I wear thick ski mittens. I choose shifters that work with heavy mittens, and my two winter-equipped bike builds (now my son is riding one of them) are 1x gearing so I only need to worry about one shifter. The smaller increments of a rear derailleur seem to make it easier to shift while wearing mittens. Also, my front derailleur turned into a rust ball.
My only mishap was that I got my chain tangled up somehow, and in the process of getting it back in business, I got grease all over my mitts, which I transferred to the bar mitts by accident. That was during my "thick sticky chain oil" phase. So I ended up having to wash them in sudsy water.
I tend to bundle up pretty good. I don't sweat a lot, so I can get away with it.