I bought this as a cheap alternative for nighttime teeth grinding - it fits well, but has so much extra material over the gums, and it's pretty clear that it is meant to be for sports only and not to wear for eight hours at night. I had no problem boiling and fitting it, and if I were wearing it to play hockey, it would be perfect - but as it says, it is meant to be a gum guard, not a teeth guard.
I've also found that this guard doesn't protect my back molars (I don't have wisdom teeth anymore), so I'm still able to grind those back teeth even when wearing this guard. So: buy this for sports, but not for teeth-grinding.