I have been flying SW since 2004, to Orlando, Ft Lauderdale, Midway, and LAX, and I have never had any of my flights changed once the schedule has been posted. I know its a pain sometimes waiting for them to post their schedules but to me its been well worth it.
USairways has gotten so expensive and Airtran changes their schedules frequently.
Of course there have been delays due to weather etc, but I have never had my flight times changed.