Headed for a long weekend in May or September - only have 3 days to visit friends. Any advice on the best /quickest/ cheapest transportation?

With only 3 days, I would fly (the drive is about 12 hours). You can fly BWI-CLT-Hilton Head or to Savannah (about a 30 minute drive from SAV). It's not cheap (though it can be reasonable if your schedule is flexible), but you have to balance your wish to maximize time in Hilton Head vs. cost.

The quickest is definitely by air and if you book far enough out it could be as inexpensive if not more so than flying.

the road distance is 633 miles

I $'s allow, fly. Look for flights to SAV and don't forget to check from Dulles. Try to stay away from Memorial Day & Labor Day. Both get very busy but both have good weekends before and after.The drive from the airport is easy but a rental car is the best option.

Infantrymom, I've driven it a couple of times, and it's a pretty long drive. About 12 hours when you figure in breaks. For a long weekend, I'd suggest flying.