I race with a mouth guard that is fairly thick and custom formed to a mold made of my teeth from my dentist office. I can eat (I don't recommend this), Drink from my water pack or a bottle, breath fine and talk (slightly slurred). There is no real obstruction, just an added layer to my upper teeth. Its so comfortable this past weekend I did my practice and then realized I still had it in my mouth when I was helping the AM class start.
I wear it strictly as a preventative measure to reduce the chance of messing up my teeth and prevent possible grinding (I get migraines after the races and it was though maybe clenching/grinding was the cause, but it turns out I wasn't so the migraines are still ongoing). I am not convinced it would stop a serious injury to my teeth and it only covers my top teeth, but I am sure it would reduce the severity especially in cases where your jaw gets slammed closed. It is basically just a very soft sheet of plastic that is vacuum formed to a mold of your teeth.
Some disclaimers:
This is all coming from a guy who also wears ear plugs when riding to reduce the noise level slightly so I might not need a hearing aid when I am old.
My mother is a dental hygienist and my sister a dental assistant at another place at the time and she is the one who got me the mold of my upper teeth and two guards made at no cost. Make sure they give you the mold they used to make the guard so you can get more made anytime (until your teeth shift too much).