It's like with everything else - suits some horses but not others, I found it a good tool. I had the short combination bit and used it on a biggish showjumper who didn't tolerate anything else strangely. It has a flat mouthpiece with a small roller in the middle, it gave me brakes, but most of all lifted the horse from the shoulder. He was the type to run through the bridle and roll on his forehand.
My opinion is that there are very few horses in this world with sufficint room in their mouths for a mikmar bit. They make a horse stop because it has to- alot pressure on the nose and poll, the horse doesnt have much choice.