We will be driving for the third time in June. My take:
I wouldn't do it every time, but for us there are advantages to both. There's nothing cool about being in a car for two 18 hour trips in ten days. But the way we calculate it, there is significant savings, and there's only two of us.
Our gas to get down is roughly $150, including tolls and such. Add two hotel rooms at $30 each (Priceline). I add realistic costs of travel (more meals, more sodas, more ibuprofen) of $90 and call our total "cost of trip" is about $300. Sure, it's not exact, but it's in the ball park.
Flying for us (we live in Central PA) from home is difficult, as the leg from State College to Pit or Philly is obnoxiously expensive. So we drive to BWI (about 3.5 hours each way), incurring about $25 in gas costs. We have to park our car for a week, cost about $60. Air from BWI to MCO RT runs around $350. We do not like being without a car in Disney, so add about $150 for rental. This would be a big savings for someone flying if they didn't need a rental car, but we do. So call our total cost of flying, roughly, between $550 and $600.
So we consider it a savings of around $300 (we have no children -- YET!).
The drive back absolutely stinks, no doubt about it. But we find the trip down nice, and we stop and 'see things.' We have our car, rather than a rental. Our packing is much easier and we can take more. And we're not set one someone else's schedule. And quite frankly, until it comes time to go back, the "seeing the country" thing is something we enjoy (although we're mostly seeing I95). I'd also throw in there that my wife doesn't like to fly, although that's never been a factor in us driving (she can suck it up once or twice a year), but an added bonus that it gives her piece of mind.
In the end, you have to weigh your savings against wear and tear on the car (we're talking about 2k miles round trip, plus whatever miles we pile on the Camry while we're there). And my opinion on the dollar savings comes from one who insists on a rental car if he flies. But I also come from a standpoint of only two traveling. The cost of flying would greatly increase if we had two kids. So would the hassle.
I think you'll get those who think people that drive are crazy, and those that drive who think those that don't are over-stating the hassle. It's all a matter of opinion (have you ever driven fifteen plus hours before). Personally, I'm looking forward to all that time with my wife and unborn child.
Pat