OK - here's the straight dope. My buddy and I did the ride for the first time three weeks ago.
1) apparently they have repaved the road to the top. it is smooth and better than a carnival ride coming down.
2) you best be ready to spend the better part of a day if you start in Manhattan. We left from the NYS side of the GWB and it was 94 miles total distance. we started early, but it got hot near midday. If you want shorter, consider driving to Piermont or Nyack and starting there or riding up the mountain and across the river to the Metro North station in whatever the heck that town is.
3) you absolutely MUST go to the top. it is not crazy steep, but it's pretty long. a very good climb in my opinion, cuz you can keep a nice rythm.
4) there is an annoying climb on 9 as you approach the park, followed by a wicked downhill, which of course means you have to gain it all back again when you turn into the park.
5) One of the best rides I have ever done. you should do it.