Just did my first ride on the D&R, the main section. Rode from the trail head at Georges St in New Brunswick to I think it was the crossing at East Millstone First Aid Squad (I'm not really from the area.) and back.
The loop down and back was about 22 miles.
Fast rolling hardpack dirt with a crushed stone dust cap on it. Was damp out but not super wet, made it slightly slick but fast rolling. Pretty smooth surface for the most part. The red/pink clay dust will gunk up your bike...plan on washing it.
I'm a fitness/explorer rider, getting back into shape and this sort of ride is just perfect for me, though likely not very exciting for MTB riders.
The scenery is woods and the canal and waterway on each side. It's nicely shaded most of the way which would be great on hot days I'm sure. It was like London fog yesterday though, still a nice day for a ride. I liked that you really have no idea where you are compared to what it looks like beyond the trail. Feels like the middle of PA.
A couple notes.
The trailhead at New Brunswick George St. has no official parking. Also the entry point there has a sort of spillway with water on it and unfastened large rocks that you basically have to walk over, carrying your bike. The rocks wobble around as you walk on them. It's not the ideal place to put-in but I had a friend with me who lives just a few blocks up from that entry point. Be careful if you do this...particularly in cleated shoes...and it's even more "exciting" after you have been riding a few hours and have to get back over it again.
Aside from that, the trail crosses a few transition points that cross the tops of spillway structures. A couple of them are cemented stone that is a bit like riding cobblestones. Not particularly hard and my advice is to keep a loose grip and pedal fast...they are a lot more bearable at say 15+mph on a driving sprint than chuddering over them slowly.
One spot of note is signed "get off your bike" and it's a steep drop over rutted dirt with loose blue stone on it on each side. If you aren't comfortable standing up and letting your bike slide around under you...heed the sign. I'm in terrible shape and still managed to ride it o.k. but just FYI. I'm a bit used to dual-sport motorcycle riding in sand and gravel so it wasn't the end of the world...but I was glad for flat bars vs drops for those few moments.
I was running 700c wheels with 40mm Schwalbe Mondials on my Ogre, which is almost a perfect combination. Much skinnier I think would be annoyingly slippery in spots, though perfectly doable. Much fatter...fine, but likely not what I would say is needed. I think the trail is perfect for cross tires, like 35mm+ if you have the choice. Not sure I'd want to ride those blue stone chutes on anything skinnier, but my skills suck.
The general trail conditions, transition sections aside and barring traffic of others, you can maintain some good speed if you are up to it. Smooth and very flat.
Definitely worth checking out and my goal next year is to do it end to end to Trenton and back which I think is about 70 miles or so. I think my current stamina limit is likely 30-40 miles...figuring in a drive home at the end. I also want to check out the feeder side of the trail next.