I've had one for a couple of years - mine's to stop me grinding my teeth.
The first one I got used to it after a couple of weeks - I was scared that I would swallow it and choke to death
After a couple of years I had ground big holes in it so got a new one in January. It was so much thinner and a lot more comfortable - however I had ground a hole in it after a fortnight! I now have a really thick one and I am suffering badly at the moment - have ulcers and generally in a lot of pain. DH took it to work this morning and has ground it down a bit so hopefully more comfortable tonight
I can't sleep without one in as I have a few capped teeth and I don't want to wake up one morning to find I've knocked one out.
Although I'm in pain now I know that in a few weeks it will have relaxed in shape and not be so tight.