Uncommon routes I've travelled (i.e., PIT-ITH) have been less than half full. Flights the week after DCA opened were about the same -- which I blame on the fact that people were avoiding booking DCA flights with the airport in a state of flux.

Flights that are on common 'commuter' routes, like Washington-Atlanta, or Washington-New York, have been fairly crowded. Some of mine have been completely full.