If you're referring to the LEAT neck braces, they don't really support your neck or head at all. They're designed to keep your neck from bending too far in any one direction so that you can't break it like that. The idea is that in a crash, if your head bends far in any one direction, the added momentum of a helmet can cause your neck to bend at a sharper angle than you normally can even if you try. This can be enough to cause paralysis or even death in some cases. The LEAT neck brace is supposed to prevent this sort of injury. Personally, I think 3 or 4 hundred bucks isn't too high of a price to pay in order to be able to stay walking and breathing. It's pretty much the same concept as a back protector, but just for your head and neck, areas that a standard back protector doesn't protect at all. The only thing I don't like about it is the way it feels around my neck. I'm used to moving around a lot when I ride, and it kinda bugs me even without getting on a bike. If you're interested in getting one, I'd say try them out at a bike shop in your area that carries them. They come in two flavors, a standard, and a carbon-fiber version. The carbon is about 100 dollars more than the standard one, if I remember correctly.
Also, BMW is also marketing the same thing, but with the BMW logo and about double the price, from what I've seen.
