Once again, I've found their prices to be locally competitive. And since I (and most gas-only buyers) pay at the pump there isn't much cause to deal with their employees, is there?

You might save a few pennies/gallon off-airport, but that would likely mean using the Dulles Toll Road (typical toll is $1.25) instead of the free Dulles Access Road, so you would have to save 12.5 cents/gallon on a 10 gallon purchase just to come out even.

For what it's worth I just called the Dulles Airport Exxon station to check on prices, but they graciously declined to quote prices on the phone - simply stating that they were competitive with local prices.

According to gasbuddy.com, prices in the Herndon/Sterling area outside the airport are as low as $4.07-$4.09 for regular. If you drive west of the airport on US-50 into Loudoun County, gas prices drop to as low as $3.99 for regular.

Hope this helps.