We visited last year over Christmas/New Year's and we had a good enough time that we are doing it again this year. Our entire week is at a BW Villa. With a solid plan, getting to the parks for rope drop, taking mid-day breaks/naps, and closing the parks at night, we felt like we were able to get a ton accomplished. Magic Kingdom was the place where I felt the crowds the most, but we often would be done and leave MK around 12pm-1pm, since we were so productive in the morning.
We will be at MK for the fireworks on the 30th and at Epcot on the 31st. Last year on 12/30, we planned to stay at MK until 2AM, but my wife decided that she wanted to head out immediately after the midnight fireworks. When we got to the buses, we were able to walk right onto a bus (that was very crowded) and we were back at the Boardwalk in less than 20 minutes. We were in bed by 1-1:30am. The only traffic nightmare I saw was the crowd of cars to get to Epcot and the TTC on 12/31. Traffic on World Drive was backed up pretty far. We had brunch at the Grand Floridian and chose to use the monorail to go from GF to Epcot, and then we walked through Epcot to get back to BWV. I don't know how much time it saved, but it was a stress-free experience.