The downhill lot have gone mad for these, checkout Southern Downhill for plenty of opinions as well as BMXtalk as mentioned. The choice seems to be down to the Leatt and I think Alpinestars do one too? They're both pretty steep price wise. As for armour, apparently it can be worn with Neck braces on the whole, although I think some of the newer suits are being made to accomodate the braces without the need for any user modification, again, this has been done to death over on SDH.
One thing I found interesting that doesn't seem to be commonly known is that you shouldn't ever wear one without a helmet (as I have seen kids doing at races, dicking about on their bikes between races). Apparently they can act as a weird pivot on your neck if you fall and cause pretty serious injuries. KIDS IN RACE PADDOCKS - HELMETS ON OR NECK BRACES OFF!