Agree with Skyegirl. I've gone at both times (Christmas week and early/Mid January) and the parks are very different. If you do go in Mid January, make sure you include at least one weekend day so that you have the possibility of fireworks and night-time spectaculars. Also be aware that it takes some time for them to take the holiday overlays off of Haunted Mansion and Small World. I think they'll be back up and running by the 14th, but it's worth looking into.
I'll be in the parks Dec 27-29 with my family. The crowds are rough, but azdisneylover has the right idea - get there EARLY, do lots of stuff while the crowds build, have lunch and take a break in the afternoon, then return in the evening for entertainment and some attractions as the crowds start to thin again.