Yes, I did it last year and had a wonderful time. I live down there part time, so I'm familiar with the routes. Mostly flat, yes, with a couple of hills (relatively speaking, they are hills) to keep us from falling asleep. Windy, though, since we're on the water.
Well-organized and quiet roads.
There's a joint RABA-NNCC ride over July 4 weekend that will be on some of the same roads as the River Ride. Check out the RABA website for details.
Oh, and one more thing, I usually post Northern Neck ride info in the Southeast section (we are in Virginia, after all), so check there, too, if you are interested in such things.