It really depends on how much traffic there is at the time. You'll probably be hitting the DC area during the afternoon/evening rush hour, so it could save you some time. We make the decision on the spot based on traffic. You don't have to stay on it the entire time. We use it for several miles and if the traffic on the "free" road looks good again, we'll get off at the next exit. If traffic picks up, we'll get on the express lanes again at the next entrance. We usually pay about $25-35 in tolls one way...but that can really vary since they use dynamic pricing. I do know it adds up quickly though. You think, "Oh, it's $3 for the next 3.5 miles of Express Lanes (I used random numbers there), let's get on!" And then the next segment is a few dollars...and the next....and the next. And then you get home and look at your EZ pass account and realize you've spent $70+ in express lane tolls for the round trip.

My husband is not very patient in traffic, so unless traffic on the "free" lanes is moving at a good clip, I will encourage him to use the express lanes.