My wife and I were there with a couple that we're friends with (our first DVC stay!!) from Jan 2-10.
Jan 2-4 it was VERY CROWDED. TouringPlans dot com predicted attendance at an 8 out of 10 - and it was right. People were still lingering from the New Years break. It was a madhouse at the MK on Friday, Jan 2. We walked in and walked right back out.
Sat/Sun (3/4) was the same for the most part. People started leaving on Sunday, but the locals were still saturating the parks.
Once Monday came around, it was DEAD! I'm used to going in June, and once in May, and this was nothing compared to those times. Toy Story was the only attraction that actually had its usual wait. We walked on almost everything else, in all the parks.
However, having seen the holiday crowd, I continue in my firm belief that the economy isn't suffering like the media wants us to believe.
And I'll second (or third or fourth) the fact that the parks were overwhelmed with foreigners! Not only those groups from Brazil, but MANY, MANY Europeans. More UK, German and French guests than I've ever encountered.
Lastly, the marathon was this past weekend, but fortunately we were leaving on Saturday when they were all hitting the parks after their 1/2 run. They didn't seem to have arrived on Friday - or at least weren't in the parks.
In summary, I'll definitely tour in early January again - but I'll wait until the NY crowds have left - and before the marathon (if possible).