I have used these for years. Definitely good if you're getting a nighttime clenching headache (worked for me, anyway). They are great for clenchers who haven't yet overwhelmed the reflex the NTI is designed to trigger. If you've already ground off your front teeth (as I have) it may be too late for this device. Even though they're made of Lexan I grind through them pretty quickly, but they're still better than any I had tried previously. http://www.nti-tss.com/
From the same designers, I think, there is a device called a B-splint. It is (almost) indestructible. But I clench and grind so hard (my dentist would like to know my lbs/sq.inch factor :D) that I have ruined one of these, too. http://www.nti-tss.com/index.php?option=com_content&task=view&id=316&Itemid=460
I am about to try another device, recommended by the same designers - the sharksplint - http://www.sharksplint.com/ - We're hoping that it will both reduce my clenching and inhibit my excursive (side-to-side) grinding.
I'll keep you posted.