I've got a ribcap, it barely gets used anymore.
They take a bit of the impact but they still hurt a LOT and basically the main aim of them is to simply deaden the blow slightly, rather than taking the full impact. We tested it at an early JJ meet with the Welshies and Q punched Locky in the head whilst he was wearing O2's, it still hurt him a lot and he nearly fell off what he was sitting on (I think it was a bin).
E.g. rather than smashing your skull and killing yourself on the pavement, you fracture it and knock yourself out.
I liked mine but it got hot in the summer and they don't stay on when you're flipping. They're a good idea for snowboarding/skiing/etc but not really for bocking. Personally I would say go with a skate helmet (definately not a bike helmet, has no protection on the back of your head), as it protects a lot more of your head a lot more efficiently, and is cheaper