I've ridden the D2R2 a couple of times. It's a very big ride. Around 900 people riding one of 5 different routes on the day. Most of those routes cross each other so it can be easy to get onto a wrong turn. But, it's also always easy to find some one to ride with. It is not a race at all, but some people push the pace for their own sense of accomplishment. The organization at the start/finish and the feed stops is top notch. In terms of terrain, the hills are relatively short, I think the longest is about 3 miles, but they are steep & relentless. The total elevation gain of 13,000 ft on the long route is true. The track is high varied, one minute you're on good pavement, the next a smooth dirt road, the next a rocky jeep track. It is the grand daddy of New England dirt road events and one of the best rides I've ever done.
I'm signed up for another go-around... it's a really fun ride with great organization, super friendly volunteers and other cyclists, and one of the most insane rolling bike shows I have ever seen (and lots of builders riding).
Personally I think there are some plenty long hills on the 180k course (what's that three or so mile gentle but continuous climb on the first half) but in general I agree with ringcycle's assessment. Keep in mind I've usually shown up to ride in less than ideal shape, and I'm a flatlander. Climbing, descending, climbing, climbing, descending, climbing. Mostly dirt. Some country highways (and Rte 2 very briefly, but way out on Rte 2 where it's bucolic). Beautiful views. Read the cue sheet, it's informative. If you ride the 180k beware "the toboggan run" it is dangerous.
Terrain is varied, you see people riding it on everything from carbon road bikes to rando/cross bikes to full-on MTBs.
D2R2 is the ride that got me hooked into riding gravel and is by far my favorite event of the season. The scenery is absolutely gorgeous and you will go most of the day without seeing cars. Most of the dirt is pretty smooth, but there are some rougher sections depending on the course you choose. Like any gravel ride choice of bike, tires, and gearing are all dependent on personal preference. There are a lot of steep climbs and fast descents so choose your setup wisely. The ride is extremely well supported with breakfast, awesome rest stops, and a barbecue with beer at the end. There is also on site camping available before and after the ride if you need it. D2R2 attracts a lot of riders, but because there are multiple course options, it never feels crowded. The organizers stress the fact that it's not a race which keeps the vibe friendly for all riders regardless of ability. The hardest part is navigation since the course is not marker. Skill with a cue sheet will save a lot of time. I cant say enough about D2R2. I'm already signed up and will be doing the 180k for the 4th time.
The descriptions you've seen are accurate. I've done this many, many times and the vibe is the best of any ride I do. The course is challenging, the cyclists and support all have the right spirit and a beer at the end never hurts.
As to questions about equipment and gearing there is no right answer. In an ideal world one would be able to ride a road bike on pavement, a cross bike for the climbs and a mountain bike on the descents. Alas, this is obviously not practical. The climbs are quite challenging so no matter what gearing you have you'll always want more.
One word of advice, carry extra tubes, chain links, chain tool, etc. Most of the day you are in the middle of nowhere on secluded roads and need to be able to deal with any mechanical that arises.
