Le Cellier is definately on my "not-to-be-missed" list at Disney. It is probably in my top three restaurants, along with Cape May Cafe and Kona Cafe (great desserts). Dinner is very hard though as Le Cellier is actually the smallest table service restaurant at Epcot.
Probably the best suggestion I can make is to try to make a reservation for lunch. The menu is slightly different at lunch, but you can still get most of the specialties, with the exception of prime rib and surf & turf. If you are going to be at Epcot for the rest of the evening, you may want to consider a 2:30 or 3:00 reservation. This is generally a slower time, so you are more likely to get in. I love doing this and then just snacking on the various treats that can be found around the World Showcase while waiting for Illuminations to start.