It's pretty great. You can show up and race whatever category you want. They often don't put hay bails in front of light polls at the finish. The prizes are minimal. But you can fairly easily become a "national champion".
Last year I won a cat 5 race. The next day I showed up for the second day of racing and registered as a cat 5 and they said "You know you can race the cat 4 race if you want?" Crazy, their site says something about 15 mass starts.
Last weekend I registered for the cat 4 race and said "Can I also register for the cat 3 race?"