I learned to make grunt calls with my mouth and they sound extremely close to the real deal. I've called in so many bucks already by doing so. In fact if the buck wasn't trailing a doe it came in every time. I can do every call that a buck makes with my mouth including the roar. And like a previous poster I had the buck roar back. It was pretty crazy!!!
The cool thing is its fairly easy to learn how to do this call with your mouth and its much nicer drawing your bow with out a grunt call in your hand. haha