i've done Ride 100 once, I had a later start time than others in the club so rode it 'solo'. I really enjoyed the route and the ride, had a chat with a few riders, stayed out of trouble, Now I like to think I'm an ok rider and having done some track cycling I'm very aware of riders around me, but Ride 100 is something else, so many riders that don't hold their line, cut corners, don't look etc.. Then you have the wannabe racers, constant shouts of 'on the right'. The hills are great fun, people stopping to get off and walk right near the bottom, this then creates holds up etc..
But all in all I did enjoy it, I wouldn't want to do it again. I was unlucky with my start time, so many riders in front, so as Tommy said, say you're fast and get an early start time, that was you should miss most of what I had. In contrast, my first 'closed road' sportive was Velothon Wales, which had fewer riders, but was similar to Ride 100, flat for the half then a couple of reasonable hills before a long sweeping descent back to Cardiff. This still had the wannabe racers shouting 'on the right' but there was more space on the road, it still had bottle necks on the hills, why do people stop at the bottom of hills to walk up. there were people walking up the Tumble, getting off at the bottom, it's 2.8 miles long, that's a long walk.
I saw nasty crashes on both rides, although the ones on Velothon wales were worse, some guy crashed on a descent at about 40mph, straight over a fence, he was not in a good way.