Anyone ever use a night guard for teeth to keep them from grinding?

The dentist recommended I get a mouth guard to protect my teeth from grinding against one another while I sleep. I've noticed that I do grind them, when I am trying to relax and fall asleep. They say that my grinding is the cause of a "disk" in my jaw deteriorating.

If you have used a mouth guard, did it work? The dentist was gonna charge around $300 for a custom one but I saw one at Walmart that looked more comfortable. It looked as if it only covers the back teeth, which is what I need. I'm just too cheap to spend the money with knowing if it really helps. Can't return a mouth guard, after all.