Someone else recommended it because it instantly solved the same problems I'm experiencing - gaping, pulling lips back, sticking his nose out and head tossing, etc.
I really like that the peanut in the middle is round rather than flat and that it's copper. Anything that will help him like the bit.
Even more expensive than Myler is Sprenger, which solved the same small-mouth problem for a friend of mine. It's like the one I posted, but they rotate the peanut slightly to match the shape of the horse's mouth. However, while Myler is $80, Sprenger runs you $125!