We went January 2008 and plan to go again Jan 2011.
My Top 10 Reasons for Jan date choice:
1. Dates limited because I am a teacher
2. Lowest costs for moderates of the dates I can go
3. Hate the heat (it was actually freezing when we arrived last time but within days it was 75F - pools were all 80F and we do swimming in cold anyway)
4. Christmas decorations were still up and parades etc for holidays still on when we arrived and then we got to see the non holiday parades too!
5. One water park open when we arrived, then the other opened before we left
6. Few rides closed when we arrived (we checked which would be closed while we were there and made sure to ride them first)
7. I need the sun in the middle of our dark and rainy west coast winter
8. Crowds strat out big but quickly disippate by theend of our stay
9. Flights are low cost
10. There is no bad time to be at Disney if you plan well!