Hello and welcome to the Dis!
We are starting to plan a 2008 Christmas vacation when the crowds are going to be just huge....I have never visited during such a busy time so I cannot compare. However, with the parks being open extra early and closing super late, with a bit of planning, I figure you can get to see quite a bit.
If this is your first visit, don't plan on seeing everything. Select favorites among each park and plan on those, if you get to do more, it's a bonus!
Having seen the decorations at WDW before, I want to return and be there again but cannot go at any other time than at its busiest.
If you have a plan, visiting at any busy time of year is doable.