A while ago I had an issue with my left shoulder: bursitis probably from overdoing OHS and other overhead pressing movements and finally assisted one arm chinups. After a certain time where I still worked out eventually my left supraspinatus tendon inflammed, leaving me unable to lift my arm. So I bought a shoulder horn from the guy who invented it, Thomas Duttine. 4-weeks return without question if not pleased was the deal and it was worth 70 bucks for me.
The way I use it is with high reps (25) and lower loadings to really engrain the movement pattern. Basically I am doing two patterns (see pictures below) and work my way up to higher weights. It does isolate the external rotators like nothing else, as proofed by minuscle weights you can manage.
All in all I am very pleased, my shoulder pain is history and I would recommend the device---it is worth a trial. Had it not worked, I probably would have returned it. However I would not rely on this device alone, 3-dimensional trajectories should be trained too (PNF-patterns), but whatever for me it did the trick. I think I would still suffer had I not bought and used this thingy...
BTW sorry for the crappy pictures