I think that depends on what kind of wind and weather they've had recently.
All dunes get razorbacks if the right winds are present, and when the prevailing winds shift to another direction, they may be smooth.
Each dune seems to be quite large width-wise, not small and choppy where razorbacks give you fits (in other words, if you're going straight up and down dunes, you'll be going over fewer hills than say, Waynoka or Silver Lake, where they're smaller and closer together)
Biggest bonus is that you don't have to worry about other people scratching up the smooth sand, you don't have to worry about nearly as much traffic (you always want to keep an eye out, even if you think you're the only one), and you don't have to worry about idiots hauling ass around your trailer at 4AM (probably anyway, LOL).

The road is mostly dirt from what I've seen on the net, for quite a few miles, but nobody has complained of the quality (some dirt roads here are smooth enough for 130mph in a stock duramax, smoother than pavement even).
Of course, Rupton should be able to answer that question a lot better than I could...Milehi and quadster have been out there a few times too.
If you go, be sure to take some pics of the parking/camping area, the road getting there, and the dunes too