This is just my opinion, since I have only heard it used in youtube videos from the '14 SHOT show.
Unless your hunting predators or turkeys from a tree stand or other elevated platform, I wouldn't waste the money.
In the "grand scheme" of things, when calling coyotes, IMO, having the sound of a grunting deer coming from 30 feet from the caller doesn't help much if the guy calling is also the shooter.
Thirty YARDS might be a different story.
Also, while I have used both fawn & doe sounds to call in coyotes, I have never heard of anyone using buck grunts. But what do I know. Hahahaha
I will admit that the sounds I have heard come out of the Ground Grunt'r sound a whole lot more realistic than the howls that the founder of Flextone, Tom Wiley makes with the Coyote Rage closed-reed "howler".
('Course, Mr. Wiley is also the same guy that states that open-reed howlers are quote: "REALLY difficult to master".)