Firstly I should explain that this wonderful restuarant isn't in Reims, but in Reiully, which is a small village perhaps 30km away. But it's so good, I would recommend travelling to it if you're staying in the Champagne region (and particularly if you're staying in "The Bed & Breakfast of the Champagne Route" which is just over the road). The Relais also offers accommodation, incidentally.
The dining here is formal, with polite, very attentive service. The food is exquisite, beautifully crafted and presented, and it tastes divine. The menu offers rich food, with traditional ingredients of the area, but interpreted in a modern way. There are a la carte menus at two prices, plus a full "Menu Degustation" which involves many courses and extensive choice. You can see the menus at http://www.relaisreuilly.com/
The wine list is very good too, with a huge choice of wines, mostly (inevitably) French but from a range of grapes, areas and prices - although don't expect anything too cheap!
The food, service and ambience combine to make this a fabulous meal, worthy of a special occasion and it makes you feel privileged to be there. Outstanding.