I've had a lot of luck over the years with grunting. Methodology-wise, it's kind of "make it up as you go along"... but there's some principles that I usually follow.
One... when you have a buck in sight and he's not going to travel close enough to shoot, grunt to stop/redirect him. A lot of times he'll act like he doesn't hear you, but then he will eventually commit (think about a guy that doesn't want to seem overeager to talk to a girl that he's convinced thinks likes). When he's close enough to be seen, do NOT grunt when he's looking toward you - wait until he puts his head down or is looking another way.
Two... when blind calling, anytime during the rut and after (till the end of the season) can turn into an encounter. If I'm in the woods, I'll hit the call more often - on plots I use it more sparingly. In blind calling, it's been my experience that good bucks usually come to investigate it right at dusk. I killed a good 8 this year on a plot in December just before we first saw chasing by grunting three series starting at 4:30. The wind for that plot was perfect - to circle and get downwind of me, he would have been in a totally open area - and I just don't think they do that very often. He slipped into the plot at dusk from an area he could smell the plot, but not me. If you have a plot that always holds does, it's a great tactic to utilize.
As far as the folks that don't have any success with it, I'd guess that there's at least four things that might be wrong. First, it's being done in an area where they aren't any bucks around (no brainer there). Second, it's being done where bucks can easily circle to the downwind side of the grunting without being seen. Third, an inferior call is being used or it's not on a proper setting. For years, I used a True Talker, but now use a Woodhaven. Some grunt tubes I hear don't sound realistic, especially on cold days. Fourth, it may be a bad setting is being used. Personally, I've never heard a buck sound like most of the call settings that read "trophy/big bucks" - they're just too deep from what I've heard in areas I hunt. Halfway between "big buck" and "young buck" is the setting I use. Like Fred, if I forget my grunt tube during the season, I feel like I'm going out ill equipped.
