We went from 26 dec, - 1st Jan this year. It was our first trip with our DD (2). We also had my god daughter (5) who is disabled with us, and she made a huge difference as we could walk on a lot of the rides no problem without queueing, and all stay together.
It was wonderful to be there, in the festive atmosphere. We stayed in a house in the marriott resort, which was perfect. We also bought annual passes, and being in Paris for a week, we took things very slowly, so spent a few hours in the park then home for a play etc....
As PP said, it definately got busier towards the end of the week, but NYE was a Saturday so I'm sure that had an impact too....
The weather was cold, but it wasn't wet or snowing, which was a huge relief. We didn't need half the clothes we packed, like thermals, as it wasn't cold enough!!
We've been since with just DD, and so not had the access to everything. We went offpeak, but it did make a difference, as we did far fewer activities per day.
With that in mind I'm not sure I'd fancy going at the same time again. We're going back for early December as our last trip with the AP's just before DD turns 3 and the price sky-rockets! but have specifically opted for monday to Thursday to minimise people!
All that said, it was a fabulous holiday, and I wouldn't change a thing about it! and of course it was absolutely magical!
and probably very cold, it snowed when we were there which was 
at first but after a day or two it was 
