Well, we were there from the 4 - 7 last year and let me tell you it was busy! (not as busy as Dec 26/27 mind you) Hot, long lines, but DL! It was hopping - maybe because of the 50th, maybe because of kids Christmas Holidays - who knows... anyways we were playing in the pools and wearing shorts and sandals early in the morning.
From what I hear, it's very bad. This year, I went from January 9th to the 13th I believe. The students were out of school until the 7th, so we missed them by only a few days. We had basically NO lines (or waits of under 5 minutes) at all of the rides for the two days we were there, but we were told by a couple cast members that just a few days earlier the line for Space Mountain and other rides had been at 3+ hours. Crazy.
The other big problem aside from no-school is that the Rose Bowl / Orange Bowl or whatever is going on, and there were TONS of people in town just for that event, and they decided to go to Disneyland since they were in town!