We were there for MLK week-end in 2006. It was more crowded, especially during the week-end days. Later in the week, the parks cleared out nicely. Of course, there are lots of rehabs during that time, too. We've missed Buzz and Test Track and other rides like it by going that time of year. Nonetheless, we are going again in January 2008, too, starting Saturday before MLK and leaving the following Saturday for our first trip home. For us it is worth the tradeoff for lower crowds as DH doesn't like to go when it is hot or crowded.
I will be interested to see what other have to say as well.