We've gone both Pop Warner week (2006) and the week after, this year (Dec 10-17.) We had great weather and low crowds both years. As posted before, it is a good idea to avoid Pop Warner (especially if you plan on staying at the All Stars or Port Orleans). We stayed at CSR and had no problem with the Pop Warner groups.
The only unusually large crowd we encountered this year, was the evening of Monday Dec 10. We went to MGM to see Fantasmic and the Osbourne lights and it was MOBBED!!!
Enjoy your trip! It is really a great time of year to go!!!