It's a matter of preference, therefore I would't buy it without trying it out first. My training partner uses it because he feels uncomfortable with a naked bar. He feels that it digs into his skin and leaves marks. For whatever reason, I don't have any issues with that and I prefer using just the bar, so I take off the manta ray when it's my turn.
If you don't like using just the bar, and you try out the device and it's comfortable for you, then sure...it's worth the $40.
I haven't tried out the sting ray for front squats, however, I do find front squats very uncomfortable. I do them anyway but it's a bigger pain in the ass than it probably should be. That said, if my gym had a sting ray, i'd probably use it.