Update: Ok, I've got 60 miles or so on the shifter. At first it squeaked against the bar tape (which was running under the shifter - could be avoided by taping differently), but it's worn in now and the squeak is gone.
So, the thing is a little big, such that you don't really want to ride with your hand around it, at least if you have small hands like me. (Might be totally comfortable for larger guys.) But, I can ride with my gloved hand right against the side of it without a problem, and often do.
As far as shifting performance goes, it seems a little less crisp than when I had the stock shifter on - I can more easily put the hub in a position where I can hear the gears are not quite right, and then give it a little tweak and it goes away, where this was not the case with the Rohloff shifter I had on for the first 40-50 miles. I think this may just be a matter of cable adjustment though, and have nothing to do with the actual shifter design, so I wouldn't assume this is them - it's really just a roller, as the indexing is, after all, in the hub.
On the whole, I give it an B+ - it would be a solid A if I could comfortably leave my hand around it in ordinary riding, but then again this may not be good for hub/shifting performance either, and would not be possible without a different design - you could, I suppose, design a shifter like the base Rohloff unit that split cleanly in half and bolted together in a flat spot on the bar, and get the best of both worlds, but alas, no one has done this yet, and then again, that would not work if the bar had any curve at all in the place where the shifter actually bolted on.
As far as the Mittelmeyer goes, most importantly, it works - I now have reliable Rohloff shifting on a curved bar, in a reasonable position where I can shift without taking my hand off the bars (as would be necessary for a perpendicular mount) and without moving my hand to the very end of the bar. It also looks good and the cable placement is convenient. Two thumbs up.