I'm going to chime in.
I use both CPAP and a mouth piece.
The mouth piece is not a cure-all miracle device, but it is a device that, if you are lucky, will give you freedom when you want freedom (travel, outdoors, naps).
1. The mouth device provides cpap-freedom with a relatively good sleep pattern for days on end without suffering the effects of severe OSA.
2. The mouth device , when used with CPAP, cuts the average CPAP pressure to less than <60% of the regular CPAP alone. AI events are significantly reduced.
3. The mouth device alone cost 2,000 (Cdn) + there are multiple office visits for compliance and follow up.
4. The process was done and is supervised by a phd in the field and the first thing the good doctor tells you is it is not a miracle cure.
5. A sleep study at around 6-8 month mark is a must for it will allow you to know how effective the device is or isn't.
Is it expensive? If it works for you, you are going to think it is a very , very cheap price to pay for freedom. If it does not work, it is expensive, sure, but you can also use it to make your jaw look like Don Vito in The Godfather
After years of trying different masks, I to wanted to go to a mouth device. I highly recommend against it. You have to learn to swallow without that crazy device in your mouth. Some can do it, most cannot. You will almost feel like you are getting weatherboarded in your own saliva. Extreme slobbering all over your pillow, face etc.
In my opinion, you must stay with a mask and get a chin strap to keep your mouth closed. Then the last problem I had to deal with was the ever present air leakage into the face and eyes. 100% compliance came for me when I added eye goggles, like the Tranquil Eye product.