Here is a raw, uncuted helmetcam video of the Glacier Bike Downhill race. This rider, Andreas Tschanz, happened to get 10th place out of 139 racers. The average speed of the race was 42mph with a top speed of 77mph.
I would love to compete in something like this. It's amazing to see how quickly they descend and how dramatically the scenery around them changes. I don't think I would want to go quite as fast though!