The results have been very variable - sometimes it works and sometimes they just seem to ignore it. I also think that at times it might have the potential to chase a smart stag off. On the other hand it can make for an exciting 20 minutes trying to call one in.
My experience is that stags which come in can do so very carefully indeed and they will often try to come in with the wind in their favour, even if this means walking well out of their way to get around behind you. I was given to believe that when using it stags would just come rushing in, throwing caution to the wind. My experience is the opposite in that it alerts them to something on the ground and makes them more cautious, even if it does attract them to carry out an inspection.
The same as Caorach says, but i was told by Adamant make sure you dont call to hard your better off doing more of a whimpy whistle so the stag thinks hes not got a big lad calling. One rut i had brilliant results, mines the sceery elk.
the best success I have had, was on ground where there was a reasonable population of deer but not what i would describe as a large population. Where I stalk now there are large numbers and my success with the call is poor to be honest, compared to elsewhere I have used it. I think that when there are a lot of stags in the area whistling away, if they were to check each other out all the time they would simply be doing that every few minutes! I think when there is large numbers they almost switch off to the challenge or inquisitiveness typically displayed by stags on ground with less of a population.
Aother point, I have used the call as late as December (out of season licence), well out of the rut, to bring stags into a shot.
I also learned a few years ago, and it was discussed on here, that you can spook younger less confident animals with it so don't use it on immature beasts too soon...
It is also a good idea to give it a go if you have a beast running away from you after being spooked. I can't recall the number of times I have watched one being bumped, then stopping for a quick look back at the whistle I am now letting him hear!!!