David
I played the Duke & Duchess courses a couple of times as an undergraduate 20+ years ago, so my memory is a little sketchy. At that time they had not built the Marquess, so it was just the Duke & Duchess
It was considered a treat, because at that time they held a Tour event on the Duke's, and the conditioning was a notch up from most other UK parkland courses in Autumn.
However, thinking back there was nothing exceptional about the architecture. The Duchess' course was extraordinarily tight, through narrow avenues of pines.
The whole set up was very corporate, and I believe it is popular with societies. I would put it in a similar bracket as a facility to Wentworth, if you have ever been there.
Perfectly pleasant in all, but nothing special IMHO