We have used them in practice for three years, and there does seem to be some evidence of reduced concussions. I can't say with empirical proof that the Guardians are the reason, but our overall program has lead to a reduction, and the Guardians are certainly a part of that.
We have never asked to (nor do we want to) wear them in a game. Given that 90% of our concussions were happening in practice, games aren't the biggest window of concern.