I only visited the White House once at Christmas, and that was many, many years ago when Reagan was president (1981 or 1982, I think). And yes, it is an elaborate affair.
The ceremonial rooms, many of which are large, are all decked out as one might expect of a building open to the public. At the time it was probably the Red Room, the Green Room, the Blue Room and the East Room at least. Don't remember too many specifics except for a large decorated gingerbread house that was sitting on a table.
As for crowds, the White House always draws them. Until they developed a Disney-like ticket system, long, long lines were the norm at any time of the year. I don't know how much longer they were at Christmas but I remember standing in line for an awfully long time that evening.
If one gets the opportunity, though, one should definitely go.