We are a family of 5 and we have decided to drive from WI to WDW at the end of this month. We are also renting a vehicle for the drive because we do not want to put the miles on our own vehicle. It makes sense for us financially - car rental (DH is awesome on priceline) $350 for 10 days, gas $300 (overestimated to make sure), no lodging costs on the way down as we are staying with his parents in Indiana the first evening. The $650 still beats the cost of airline tickets by at least 50%. Additionally, it gives us the opportunity to stop along the way at a few places we want to visit including the Florida State University football stadium, to stretch our legs and for a photo op.
Our drive will be a total of 22 hours each way and like I said, for us it makes sense. The kids are easily occupied by electronics and DH and I will get a bunch of books on tape to listen to while we take turns driving. This will be our first drive to WDW, but we have driven from WI to a family camp in Maine for the past 2 years and don't regret that decision so I am fairly certian this will go well too.
Enjoy your trip!