We were going to drive in October (we usually do), but SWA came out with $49 fares. At that price, (reduced further with our Capital One Miles), it was cheaper to fly. That made my wife very happy. 3 hours instead of 20.
With our family of five, it is usually cheaper to drive. Without using our miles, the breakeven for us would be between $4.50 and $5.00 per gallon (although it would probably be lower, cause I don't mind paying a $100 or $150 premium to spare the long car ride).
In the end it will still cost us more to fly, as we added on 3 days to our trip, so our accomodations and food costs will go up. Totally worth it to me.