My wife and I went to the Greenbrier for my wife's 40th birthday in November - we left our daughter at home with her grandparents.
The resort is impressive in its size and scale. The restaurants were very good - not the best food or service you've ever had - but very good. The activities available to us in November were good - we went horseback riding and visited the casino and the shops too much. But we are really looking forward to returning in late summer to enjoy the swimming, tennis, golf, croquet, etc. This resort is straight out of a post card.
I would skip the Windsor Club. We stayed there. The service there was very attentive - we were almost always the only people in the club. And they surprised my wife with a fantastic gift at dinner the last night. It had tasty food - but really just fancy appetizers - and open bar on wine and alcohol. So why skip it? Because the resort is so expansive and the restaurants are so nice, why would you want to eat all your meals in the club? We wanted to experience The Restaurant and the breakfast buffet, so we did, skipping the offerings in the club. And if you have the means to go to the Windsor Club, why do you need to skimp on meals? All in all, it was nice, but not worth it.
To end - I am looking forward to going to the resort again with my wife and daughter and enjoying all the resort has to offer in its prime seasons.