I used to ride my old horse in a flash because she would open her mouth and bolt and it worked well to stop this. My current horse goes worse the more tack and interferences he has on so I don't ride him in a noseband at all so it depends on the individual horse and the situation
I always ride in a flash and highly recommend one, but make sure it's not too tight!
Keep it kind of lose or else the horse might start to cough
Can be useful, but it's important to make sure that they're not tightened too far and also that the reason the horse is opening its mouth isn't pain related before you add one. Personally I wouldn't use one unless I had a really good reason to.
I road my guy in it and he went will with it.