I've done the 555 several times in my 911. I've also done most of the good driver's roads in California for comparison. The 555 is absolutely crazy. I've done it twice without running into any traffic in either direction - a solid 60 miles of fun. What makes this road different is all the blind crests. You don't know until the very last second when you reach the crest if you've got a left, right, or straight on the other side. You need really quick reaction time (and a bit of faith) to drive it quickly. It got to a point where I got tired of turning the steering wheel and wanted to get off the ride!
edit: the speed limit on this road is 55MPH. I struggled to go anywhere near the speed limit!