I've used them. They will definitely help your form. You will know what a clean release should feel like after you use one.
However, they must be used frequently or you will lose that muscle memory. When you use one for the first time ...or use it after not using it for a while....you will feel a definite collapse when you release. It trains you to maintain that backward tension.
You may think you have good back tension, but if you've never used a formmaster, your back tension is not as good as it could be.
The downside is that they are so expensive. If you are handy you could make one.
I recommend the static version because you don't need to buy the rubber tubing. The static version is made with nylon cord, and the length is set so that it just stops an inch or so after you release.
Here is a link to the patent which has diagrams and descriptions should you want to make something similar:
patent