I was also on that century that Ronjon and mkadam were on, it was fun and not too hard. I think the difficulty is determined by the hills. Next Saturday some of us are doing a ride called Sea to Summit, with 12000 feet of gain in 87 miles.
I have done centuries since 1988 and have never done a double. Never wanted to spend that much time in the saddle. If I was sure I could finish in 12 hours in daylight, I might try it, but I don't think I ever want to be on a bike for more than 12 hours and I don't like riding a road bike at night.
My goal is to have fun and stay in reasonable shape and centuries are part of that, but I see no reason to extend myself to a double, triple, brevet, Paris-Brest-Paris, RAAM, whatever.