One thing about the form masters is that there are two kinds. I believe the "Rigid" form master is the one that's supposed to be used with trad bows and the other one, which uses a surgical-tubing/bungee type cord, is supposed to be used with compounds.
I have the Rigid form master and use it indoors at home without an arrow whenever it's raining out and I can't practice outside in the backyard. So in that sense, it's great as I can still practice my release without dry firing.
Aside from the much lower cost, I think home-made form masters can be better than the one I bought in that the one I bought comes with metal adjustable screws to adjust the length of the cord. And those screws can hit your face or arm pretty bad and break your skin. So you can either wear an arm guard on your drawing arm to prevent the metal screw from cutting your arm and wrist up, or you can undo everything, cut the cord to length (and hope you measured correctly, and then re-install it (which makes you think you could have just made one yourself for much less).
Despite that, I really like it and would recommend it to anyone wanting to add it as part of the shooting/training regimen. It definitely lets you know if you're overbowed. I can easily draw my right-hand 45# bow using only my back muscles, but when I switch to my left-hand 45# bow, I really struggle when drawing only with my back muscles.