I know it's considered to be a useful tool at all levels. Rod Jenkins told me once that he planned to use the Formaster to hone his draw for a tournament he was planning to enter. He was also using it in the class of his that I was attending to teach us the basics of back tension.
As Arne says, you have to transfer it to your normal shot. Using the Formaster is kind of like a tug of war: you just put everything you have into hauling back with your back muscles. When you're shooting an arrow, you probably want to hit something, so you're trying to be a little more coordinated. Mostly, we fall into the trap of trying to be too delicate when we're trying to hit something, and then we collapse. The Formaster helps us to beef that up a little.