I've run the Amoeba version (same thing) on a horizontal mount - in this case you had to loosen the adapter and remove it with the wheel. Never had it on a true dropout.
The adapter worked fine, but you could get it to shift (rotate) if you held the brakes and yanked the bike backwards.
I've use the Brake Therapy Adapter, The Woodman, and the AtomLab versions. The BT adapter is the nicest, and the Woodman is right up there, but both require special or modified hubs. The DiscBrakeAdapter version works with most hubs as is, but does not fit all frames, and was not as stable as the BT or Woodman (but it was fine). The AtomLab version required you to shorten the axle ~2mm (or bend the frame) and had a tendancy to rotate, which can be cured by adding a torque arm.