Simunitions are an expensive proposition. I'm aware of at least one very able instructor using Airsoft for Force-on-Force classes very effectively.
My group uses quality Airsoft pistols in our Basic Handgun class. They shoot Airsoft in the classroom before live fire on the range. It's proven to be very helpful.
"It is long been a principle of ours that one is no more armed because he has possession of a firearm than he is a musician because he owns a piano. There is no point in having a gun if you are not capable of using it skillfully." -- Jeff Cooper