It takes a bit of practise and you have to blow quite hard to make it sound like a sika stag. You need to pinch the mouthpiece quite tight, I use my fingers for better control though I think the idea is to use your teeth, to get it high enough pitched - otherwise you sound like a demented duck. Get over to youtube and watch a few videos of sika calling and then practise a little bit. I've never managed to make my call sound exactly like a sika stag as the call is a little more reedy whereas the sika are more squeaky, for want of a better description. However, this doesn't seem to impact upon the effectiveness.
I've found that if you get any water at all into the call it stops working and be very careful if you decide to wash it out as there are actually two parts and the water gets in between the parts and this must be important to the function as it stops working and will not start again until this trapped water dries out which takes about a week.
In terms of effect I've found it very variable - I've had stags come running in and I've had it ignored. For the most part my experience is that the stags (though mine are very wary as they are poached a lot etc.) don't just charge in to the foot of your highseat but rather they can come in rather carefully. You need to have your eyes open as often they will sneak in to try and get a look at the opposition and when they don't see anything, or see something that worries them, they will then sneak around the wind to get a sniff. So, don't just blow and expect not to have to work for your supper as my experience is that the call will only bring the stags in to around 150 yards or so on open ground, and they will use all available cover to get there and not just wander about in the open.
Because they always seem so keen to get your wind I'm going to try calling them with a large open area downwind of me - so if they want to smell me then they have to come onto the open. This is sort of the opposite of what you might expect to try but I know for certain that when calling stags I've called in animals that I've never got to see as they have come into the thick cover behind me to get my wind when it is natural to assume they might come onto the open ground to try and see me.
I know other people have had more dramatic results with the call and my deer are very wary which might reduce the effectiveness but hopefully my observations are of some small benefit.
