I don't think I've ever been to a concert where I've seen the opening band and didn't stay for the main act. But the ticket was free and the car ride seemed better than the walk to and then ride on the metro at 10pm. I did catch the first few songs of Josh Ritter's set (the headliner) and he's no slouch.
There was a decent crowd for the opening band, the Low Anthem. I think this is the third time they've been through the DC area in the past year and a half or so (unfortunately I missed them the first two times). Pretty full sound from a trio... I'd assume the fact that they are currently touring with 15 instruments might have something to do with that. And 1 of those 15 instruments didn't include the cell phone Knox Miller held up to the mic to create a few seconds of feedback at the end of one song. The Low Anthem impressed. And they got a few sentimental points when Jocie Adams broke out the clarinet (I had a crush on a clarinet player in 4th or 5th grade... bitch broke my heart).
NPR has their set from the 2009 Newport Folk Festival.