As Steven Curtis Chapman put it, "If this doesn't light your fire during Christmas, then your wood is wet!" It is an unforgettable experience that moved me to tears the first time I witnessed it. I have the CD for it narrated by Phylicia Rashad, and can listen to "Rejoice with Exceeding Great Joy" and "Do You Hear What I Hear?" over and over again during the holidays.
As others have said, I strongly recommend the dinner package if you have the $$. Not only does it help get you a seat in the theatre, but it gives you a dinner priority seating, which can be very hard to come by this time of year in some of the best Epcot restaurants.
Be wary of some of the Tier 2 dining options if you're picky about your food choices. Those often limit you to a "special Candlelight menu" without many options, and are also in the not as popular restaurants. I've had excellent experiences at Le Cellier, Chefs de France, and L'Originale Alfredo de Roma. At the San Angel Inn, our choices were very limited and we were treated like second class citizens and not given the same service as others dining from the menu.
Tier 1 dining is buffet style (Biergarten, Akershus, and Garden Grill) and least expensive, whereas Tier 3 is fine dining from the menu in the most popular restaurants (Coral Reef, Mitsukoshi, Le Cellier, Chefs de France, and L'Originale Alfredo de Roma).