These are an awesome replacement for my lost dentist-made nightguard. I was concerned about other reviews saying they were too small (I'm a pretty big guy) but they work wonderfully. They are small enough not to get in the way and large enough to protect all my teeth. They only actually cover a couple of teeth in the middle of the jaw, but hold the jaw apart just enough that none of the other teeth can reach each other. There's no way I can grind any of the teeth I damage. It doesn't interfere with my tongue, or rub.
It does move around in my mouth a little, but doesn't come loose on its own. The instructions are very clear about not using one for more than 3 nights and to discard at the first sign of damage. Each nightguard comes in its own sealed pack, making them great for travel or emergency use too.