The health risks of riding coasters are very minimal. You are probably more likely to die of sunstroke while you are at the park than to die of anything to do with any of the rides. You are definately many times as likely to die while driving to and from the park. (About 20 times as likely.)
A couple of articles have been published by doctors who treated someone who had a blood vessel pop in their brain a week after riding a coaster. They probably treated many more people who had the same thing happen a week after playing golf. Actually, positive G's on a roller coaster will somewhat reduce the blood pressure to the brain, since your blood is trying to get into your shoes.
The one study I saw that had some real science in it referred to some hazards associated with getting bent over the lap bar at high G's and then hitting a bump. This is probably why some of the new lap bars come up higher than a simple bar. Dig around for this, it related to the big woodie at SF Fiesta Texas.
I strongly suspect that most of the people who are concerned at G's on roller coasters have never ridden one to understand what the real issues are. Your brain receives a much greater insult from hitting an OTSR than it does from 5 Gs. Some special circumstances such as the bent over the lap bar thing can also be hazardous.
As far as deaths are concerned, The number one cause is rider stupidity. The other major cause is mechanical failure. Deaths attributed to properly riding a well maintained roller coast are extremely rare.
