We have gone the week following Christmas through January 4-5 the last two years.
If you want to do it, prepare yourself in advance that you will have to deal with heavy, heavy crowds. You can't see a lot in terms of what's right around you - there are people everywhere. There will frequently be waits for even bathrooms if you're female, unless you are savvy enough to know where the more hidden ones are.
As for actually riding things - go early or go late. Anything between 10am and late evening is madhouse material. If you go to rope drop, you can ride some things before people manage to get into the parks (and that time of year, rope drop can be 7am or 8am). And like a poster above me mentioned, as long as you don't have small kids and you can stay up late- the hours after 11pm can be quite good as the standby lines are shorter during and after the final fireworks show (that time of year, usually that's 10pm or 11pm except for NYE itself when it's midnight of course). MK often stays open until 2-3am that week. They close for 4 hours to clean it and then open back up at 7.
It's a very different experience than going any other time of year. We filled a lot of our afternoons outside the parks - for example we went on a monorail/boat tour of the resorts one afternoon, to see all the decorations. Twice that week we flat out went offsite (one night we just couldn't face dealing with MK again, and decided to see a movie, but given the traffic and congestion problems at DTD and the AMC there, we left property went to the Pointe for dinner and a movie).