I bought the Ab king Pro about 5 months ago now and there are good points and some bad.
I will highlight all in this article.
First try: the motion felt a little unusual and uncomfortable but it was easy to get used to.
I did about 150 repetions.
About 50 at different inclination levels.
I soon found out that it wasn't necessary to change the inclination of the bench as it doesn't make any difference when you can manually stop yourself at different positions.
It hinders any progress if anything.
I woke up the next day and my abs felt like they had gone through a thorough workout.
So yes the Ab king pro does work.
If you do enough reps it will tone you up.
Every single day after this I used the Ab king Pro to find out I was not feeling the same effects.
I was doing about 100 reps at different inclinations and many short reps like crunches throughout the day.
I have been doing that ever since.
Never have I again felt a burn.
My abdominal has built up a tolerance to this motion immediately.
The same full inclination motion seems to do nothing but stretch my abs as the majority of work is done by the arms! I tried doing real sit-ups after a couple of months to find that 20-30 sit-ups was an amazing work out compared to the Ab king pro.
30 crunches on the floor beat 150 on the Ab king pro.
Given the Ab king pro is a much easier way to do sit-ups but it takes 3 times the time to get the same effect because you use your arms to push the bar up not your stomach.