IMO it's just okay, although they may have beefed it up a bit from when we last went a couple of years ago. We regularly go to Boo at the Zoo and it's great, the December event is nothing close.
Do you have a car? There are some other great alternatives in this area. In addition to the national Christmas tree and Pageant of Peace on the Ellipse, the Botanic Gardens near the Capitol has nice displays, and Brookside Gardens in Wheaton, MD is fantastic (among many other options).