anecdotally interesting, but statistically, rollercoasters are safe. That's backed up by the CPSC report on the linked site. The vast majority of injuries are from guests being stupid/showing off/drunk.
Everyone who has ever worked an amusement park ride has a lot of hair-raising stories of crazy stuff, including minor mishaps, personally i've seen a couple dislocated shoulders, a bunch of bumps and bruises, and some very badly faked whiplash "injuries" with threats to sue my arse. Every single one of the actual incidents were caused by reasons noted above. Yes there are freak accidents, poor construction, but you're still more likely to be injured in a car accident than on an amusement ride so long as you don't be a doofus.