I walked in October 2012, finishing on Halloween. Weather was lovely, cold mornings, sometimes below freezing up in the hills, even a hint of the first snows but temp builds during the day. Peaks around 4-5pm where it was very hot some days, then falls away rapidly. Lovely cool evenings.
Downsides, bigger chance of rain. Had some very big downpours. Also, albergues can be a bit unpleasant. They get really stuffy because a lot of people won't want windows opened because of the cold night air (I'm told that's a very European thing) so that's 50 watts of heat each person is giving out with nowhere to go and the heating will be on, too. Drying damp gear and boots can be a pain as well. Even just doing laundry when the air is damp is a chore. Colds and viruses were rife, I kept away from albergues for a while because there were so many reports of people getting sick.
But it was a lovely time of year. You got to see autumn turning right in front of your eyes as you moved west. I remember it started off green at the beginning, all the leaves were brown by the end. The last couple of days approaching SdC was incredible. Lots of woodland, it was windy, leaves blowing and rustling, so atmospheric.