When gas prices are low, it's cheaper to drive, especially if you have extra luggage you want to bring with you. It does eat 2-4 days out of your vacation time though.
With kids, it's a 1 1/2 - 2 day drive each way (about 22 hours from the MA/NH border down 495 to MA turnpike, Jersey turnpike, Tappanzee bridge, loop outside of DC, then straight down 95 to 4.) with rest stops, gas, and a motel night.
With adults, going straight through, it's just a day if everyone takes turns. Just make sure people are sleeping enough before their turn to drive!
Also, getting an insanely early (3-4AM) start helps you get through the morning city traffic hitting DC after their morning traffic.
Of course, we did this years ago, so I might be a bit off with remembering the directions/times.