I have had mixed results as well. Anywhere from bucks running to bucks walking right up to the base of my tree.
One thing I have noticed is that they seem to pin point where the call came from. Once I have called it is much more difficult to stay undetected.
I also caused a fight between two small bucks using a grunt call. I was between them when I grunted and the buck who was with the doe thought that the other buck had grunted. He left the doe and came back and got into a fight with the trailing buck.