For downhill specifically, I always rode with one just because I was sort of made to (the wife is a nurse at Vancouver General and has told me there are always a handful of riders that are air lifted in with neck injuries every season). But I know there's info out there that states they don't really do much to actually protect your neck. Even wearing a neck brace, I always had to hope I would never tumble and hit a rock with my spine or ribs.
I started the season without one. Man, talk about liberating! So much more range of motion and comfort, but I got a new one just because of the reason above. It sucks.
I can see the benefit if you crash and your head is push forward: the chin bar will bottom out on the chest portion of the neck brace no problem. That's fine. But I don't see how the helmet will bottom out on the rear portion of the brace if you crash and your head if forced backwards. I tried pushing my head back but there's no way to get the helmet and brace to contact (yes I've played with the angle adjustment pieces). I figure if I hit hard enough for this to actually happen, my neck will be wrecked anyway. As for the side to side motion...well, it just depends if you crash and the side portions of the brace line up with the edge of the helmet. If not, you're f-d as well.
I started the season without one. Man, talk about liberating! So much more range of motion and comfort, but I got a new one just because of the reason above. It sucks.
I can see the benefit if you crash and your head is push forward: the chin bar will bottom out on the chest portion of the neck brace no problem. That's fine. But I don't see how the helmet will bottom out on the rear portion of the brace if you crash and your head if forced backwards. I tried pushing my head back but there's no way to get the helmet and brace to contact (yes I've played with the angle adjustment pieces). I figure if I hit hard enough for this to actually happen, my neck will be wrecked anyway. As for the side to side motion...well, it just depends if you crash and the side portions of the brace line up with the edge of the helmet. If not, you're f-d as well.