Yes, everyone who has attempted it has broken their legs and is now unable to walk 
There is a risk of injury with every sport, same goes for bocking. As long as you take it easy, wear pads, and only go as far as you feel confident you should be fine. Most injuries occur when people either push beyond their ability or they simply make silly mistakes, and I would say the majority of Injuries I personally have seen have been broken wrists from people not wearing wristguards, or broken collarbones from people falling odd.
I've only ever had two major injuries occur whist I was out bocking, which are:
I was in Cardiff in July '08 and one of the guys dislocated his elbow from falling onto his arm from a large height. He took it very well though, and was back bocking in a matter of weeks.
I was in Cardiff this weekend just gone and a kid on his first day broke his leg because he landed wrong. He also took it very well, but is going to be off bocks for at least 2 months.
To add to Cardiff this weekend, people there have been bocking for over a year and were doing backflips and crazy stuff, and they have never broken anything. Yes accidents do occur (like this weekend - He wasn't even pushing hard, he just landed odd), but it comes with the territory, and as long as you take it easy and learn at a reasonable pace that you feel confident and safe with, you should be fine
