The mouthpiece design is really genius. I feel like the Myler's really studied the horse's mouth structure well, to create a design most comfortable and effective for the horse.
I don't use one anymore now. My horse is beyond happy in his current bit, so I'm not going to change.
I previously used a Level 1 Myler. My horse didn't like it at all. They tried to accomplish "tounge relief" so much, to where the bit just hangs in their mouth, and it just doesn't sit right. He was constantly leaning heavy into the bit, trying to get it to sit higher in his mouth. If you get the mouthpiece adjusted right, then the sides were pinching his lips. If you get the sides adjusted right by his mouth, the the mouthpiece with hanging way low. Just not a good design for most horses.
If I bought another Myler, which I've thought of doing, it would be a level 2-3. It's less curved, and designed better. I've heard Dale Myler himself, say most horses really don't need a Level one, and that most are happier in the 2-3 mouthpieces.
So, I really like Mylers, I just wish I'd gotten a different kind than I did. If you buy one, I'd suggest don't go with any Level 1 bit, I haven't really seen a horse like that well yet. My trainer swears by most Mylers, and I've seen a lot of too-mouthy horses in level ones.