From discussion I had with MJK at Ally Pally, it might be a bit like helmets. The best one is the one that fits you properly. Get out there and try several different makes. See what feel right. That's the advice I had.
:)

havent got round to gettin one yet but was strongly advised to go for the knox one

Don't know how tall you are but I'm 6'5 and couldn't get a Knox to fit me very well (I think they used WeeJohnyB as a model :sing: ). I use one of the Dainese vertabrae type. Very comfortable in wearing but I've not had to put it to the test big time (couple of lowsides and that's it).

I have the Dainese vertabrae type and it looks like it would be up to the job. I dont want to test it though. Its pretty comfy but secure feeling. Alpinestars make a good looking one too

Out to get new protector soon and I read the "ride" mag comments:-get one that does not go as far up as your neck and not covering your coccyx

recommended choice was Davies Odell Forcefield - but personally I'll probably go for a knox.

Knox currently has knox assist accident cover (albeit with limitations):
knox assist
Could be tempting

I've "road tested" my Dainese flexible back protector and it seemed to work fine, it took an impact which saw the back of my Arai nearly ground through the outer shell !!. Maybe I actually hit with my head first ??, could explain a lot :lol::P