I have a buck who, according to the man I purchased him from, has zero fainter in him.
But he faints. And he looks just like the pics of the 'silkie' myotonics (with slightly shorter hair

).
He's very calm, doesn't climb the fences. He doesn't butt things, and so far hasn't broken ANYTHING.
The only thing I don't like about him is that he also is a bit afraid of stairs, so getting him out of the barn can be a bit of a hassle.
I probably wouldn't have bought him if I'd known he was a fainter, but I don't regret it at all. He's a very well behaved boy.
Then again, I make sure I don't scare him, either, since it's obvious to me that he doesn't like fainting (which is why I think he is so very careful). I don't... morally like the idea of causing an animal to have an uncomfortable situation for my own amusement, so I go out of my way to keep things as calm as possible for him.
I wouldn't discount the breed based on why they were bred. He has been a wonderful change from my last buck (who broke everything he could and chased me out of the pen numerous times before he was finally shippped off). As a matter of fact, when I consider the fact that I should be 'breeding up' with a purebred buck, the fact that my current boy IS so well behaved is the holding back point for me. His calm, quiet demeanor fits in perfectly with my little homestead.