We live about 30 minutes from Six Flags America. We had season passes last summer and went quite a few times. The theme park itself is pretty small. If I remember correctly there are 4 metal coasters, 2 wooden coasters, a few water rides and a couple spinning rides. They have a great variety of kids rides if you have small children. Overall it wouldn't be worth a specific trip, but would make a good side trip if you were coming to tour DC.
We took a one week trip last summer to Philadelphia and visited Six Flags Great Adventure. It is technically in NJ but is very near Philadelphia. That was a much bigger and much better Six Flags in our opinion. More coasters and rides for adults and bigger kids, but still quite a few kids rides. They also have a drive through Safari that had great animals to view as you drive through. A giraffe came right up to our car and looked down into our sunroof. It would make a great day trip from a Philadelphia vacation.
If you are choosing between the two, Great Adventure is a much more thrill filled park (more similar to Six Flags in Atlanta). Both have water parks too, but we never went to either so I can't add anything about that part of the parks. I don't know that I would make a specific trip for either, but if I was making a trip to either DC or Philly anyway, it would be worth a day's visit.