I picked the Helite Turtle 2 on the basis that it offered excellent spinal and neck protection, one of the few that effectively immobilises the head in the event of a sudden acceleration or deacceleration, once deployed. This mode of injury seems to be amongst the most common for neck and head injuries, so there are some valid points made by Helite in why they use the protection standard they do in place of EN 1621-4, a point further noted by Dan Read of the Air Ambulance in his response:
"I spoke to Dan Read, an Air Ambulance Advanced Paramedic with MAGPAS who told me that one of the greatest risks to spinal injury is hyperextension of the neck (backwards movement of the head), and the Helite gives very good support in this area, also limiting movement of the helmet to the sides and back."
That and the fact it uses a very substantial passive back protector too, and has some decent coverage of the front, albeit not certified to the same level as EN1621-4.
I'm in the camp that it's better to have one than to ride without one as the chances of it saving you from serious injury are good. I deliberately chose not to buy one with any form of electronic trigger as I simply don't trust them. Mechanically tethered is fine with me. The greatest risk seems to be in a rear shunt thought where a mechanical one won't deploy unless you are thrown from the bike, by which time the sudden acceleration acting on the neck may well have done serious damage. I'm especially cautious of rear shunts in traffic though and try to use frequent all round observation to manage the risk where possible.