As others have mentioned, it does require getting used to. Guessing CPAP does also. It was easy to put together and use, and as suggested, it does have a learning curve. Trying to get a good enough seal so it can create a sufficient vacuum to draw your tongue forward takes some reprogramming of how to place your tongue while you are sleeping. Having something in your mouth turns on saliva generation. You will find yourself drooling out of the corner of your mouth when the vacuum seal is not sufficient.
For me, I wanted to solve the seal issue so I first tried wrapping an ace bandage around my chin and top of my head to help hold my jaw closed. It was adequate but wrapping a long bandage around every night was not ideal. I tried a bandana, but it was too short and tended to slip off the top of my head. Ideally, you would need something that could go around the top (holding the chin up/closed) and around the crown of the head to keep it from sliding off the front of your head. Without making your own or buying one, you might not find a solution here.
My second option was tape. I started with a basic medical tape (about 3/8" wide and on the donut-shaped dispenser). I placed about a two-inch strip on either side of my mouth, halfway between the tube and the corner of my mouth. Getting the right angle (towards the nostril or towards the cheek) will likely be a personal preference. Facial hair will impede the success of this. I found that the tape eliminated the drooling issue and helped create a more consistent seal. I was achieving a 92-95% seal time each night. The tape option wasn't the best if it got a bit of moisture and wasn't overly strong so sometimes I was using three or four strips. I later tried a few different types of tape. They were about an inch wide. The first was too difficult to tear. The second was easy to tear (but not straight) and actually stuck a bit too well.
For those considering iNAP I would highly recommend using tape for increased success. The biggest downside to tape is that once you get it in place you cannot talk and doing so means removing it and it may not go back without getting fresh pieces. Heaven help you if you have to sneeze! Luckily that never happened to me.
The following critical review of iNAP may be identical to CPAP, I cannot say for sure since I have not used CPAP. If you have a cold, stuffy nose, etc, you cannot use iNAP that night. If you cannot freely breathe through your nose it doesn't work since the mouth is entirely sealed. This may mean that, if it works for you, some nights you won't get as good of sleep. I am a light sleeper and kept waking up. Possibly as the suction was failing or adjusting, or maybe my overactive brain wondering if it was sealing right. As a result, I wasn't getting a solid night's sleep. I also suffer from depression and there were some nights I just didn't want to bother using it. For these reasons, and the fact that I did not see a marked improvement over when I wasn't using it, after about four months of having it, I decided to cancel my membership and return it. Boxing it up today.
For those with severe apnea or aren't light sleepers or those who can adjust to it better than I did, it MAY be a solution and alternative to CPAP.
I'll monitor this thread and try to answer anyone's questions about the product, based on my personal experience.
The best of luck regardless of what route you choose.