They are good for horses that can get a bit keen in a normal snaffle, as they just cant lean quite as much on them, and the rolly bits encourage them to mouth. However, personally I dont like single jointed mouthpieces much, and they are very "square" so have a very obviously nutcracker action, as there is no curve in the arms. Neue schule have just started doing one with a lozenge in the middle. A firend used one for a bit of help XC with her ex racer.
My sister and I used it on our old horse. Although we had a full cheeked copper roller. Glen wasn't a strong horse but could be very cheeky and it just gave us a little more control than an ordinary snaffle.
I am going to try it on my baby, she is in a dropped ring snaffle at the mo for jumping and she really leans on it and sticks her neck out so tought I would try this to stop her leaning
A friend used it on her 15hh anglo arab who leant on the bit and got a bit strong sometimes. It is good for horses who lean or take hold of the bit with their teeth.
i have used this bit though ive been told to be careful with them because someone i know someone who has had a copper roller bit disintegrate in a horses mouth and fall to pieces twice before with two different copper rollers on two different horses
ive never had a problem but it has slightly put me off using them