Ok, yesterday was my first day trying it out. It worked really really really well. Breathing was no problem at all and it was comfortable. I actually think since I had something to bite on I could loosen my grip and squeeze on the bike in certain sections making me faster
I ended up getting a crash in, and hit my mouth somewhere along the line of it, guess what? All my teeth in tact and no holes in my mouth from the backets grinding
Downsides: They get alot of mud stuck into mold when you take them out, not a problem if you like eating dirt, and its a pain to wash out. Also you tend to salavate and sometimes get a gaggy reaction wearing it.
Conclusion- If you are going on anything more than just a sunday ride with friends, you should probably get one. 3$ is worth the investment, mine already save my teeth. If you want your pearly whites to stay pearly white rather than black and falling out, then try it, you won't be dissappointed