I've had one for several years.
I had managed to injure my left shoulder doing squats on a smith machine (the wrenching movement twisting the bar with heavy weight caused it) and then didn't take care of it and got a 'frozen capsule'.
Basically I couldn't move my left arm much above parallel to the floor, and it was very painful. So you can imagine how difficult it was to squat.
The Manta Ray made it possible and even help me work that adhesion loose.
It does take some care in positioning, and you absolutely don't want it on bare skin. If it moves, it doesn't slide on the skin, it take it along! So a shirt that covers the area is a must
It take a little getting use to because it holds the bar up higher than is typical for squats.
I have a few problems with that same shoulder from time to time (a bone spur I think, but I know what to avoid mostly) and if there is any inflamation that causes pain I will use the Manta Ray for squats, it's much easier on the shoulder joint.
I really can't see how it would be cheating. It's the same movement; maybe a very slight change in the orientation of the movement, but not much.
I do get some mild bruising on my traps with a lot of weight, but it's very minor