We are getting our first reports back from those shipped out and the feedback was what I expected. Most think it's awesome. A few think their thumb slips off the bottom.
If your thumb is slipping off bottom, that is by design and means you are not putting pressure in right place. If you push down, it will slip off. You should be pushing with your thumb right in middle of gas pedal towards the bore about 45 degrees, then grip very hard with left hand just like a hammer or baseball bat. That's the OPP grip it was designed for.
I figured this out myself during prototyping and when I was lazy with the thumb it slipped off for me too. But grip it like described above and you will be surprised at results.