If a field-test/ report is of any use to anyone, I used one of these el-cheapo master cylinders last summer in the restoration of my K100. I'd used them before on a couple of other projects.
The original BMW one and the rear caliper had been given to a friend circa 1999, and when I looked at the price of a new one I first squinted at the screen, then I winced, followed by a sharp intake of breath, and finally a right good chuckle.
A wee bit of footering with a bracket to get it mounted (since I don't use the original footpeg mounts) and the action was champion.
I've put about 1000 miles on it now and the operation has been perfectly fine. I only really 'trail' the rear brake in either heavy stop, or complete stop scenarios, so I couldn't say that it's had much in the way of abuse, but it works as it should.
I can't really comment on the long term efficacy of the gadget, as I can't presume that it's made from highest-grade unobtanium, but it hasn't crumbled into pieces yet.