I am a shallow sand guy. 2-3" is about right for me... any less and I get bare spots when the fish like to move it around. I like that it will buffer, swap some phosphates and keep the nitrates at zero and also allow me to have some cucumbers and conchs to keep it cleaned up. I would not run a tank without it.
It can move around at first, but once it gets some bacteria on it, it should mostly stay put.