Haven't been on here in awhile, but I've been reading this, so I'll just give my opinion. I bought the EVS neck brace the week it came out. I bought my size according to weight (can't remember if its S or M atm, probably S), and I have a real hard time getting it to fit properly at first (probably wasn't tested or designed to fit well on females..). I had to tighten the most it goes, and I had to add a thing to connect the front of the neck brace to my chest protector, because it comes up and bothers the soft spot of my neck. However, wearing it while riding after that change was made it doesn't even feel like it is there. Yeah, sitting at the line or whatever and you try to turn your head certain ways its hard but I don't notice while riding.
Recently I went trail riding, and used a hydration pack thing, and it was just to bulky with everything on, so I rode without it. It was a nice change.
Now a week or so ago, my boyfriend who has the Alpinestars bionic neckbrace, took a pretty nasty spill (no one actually saw it and he got knocked out) and it was clear by the destruction of his helmet that he went headfirst into the ground. The result was a broken shoulder and a concussion. He hasn't really had other bad crashes to compare to if whether or not he'd have gotten other broken bones.
So here, more experiences to draw opinions from. I will continue wearing my brace if I'm out on a mx track, in the woods I probably won't. The neck brace concept I think does need some more testing..