It seems to me the biggest issue with I-66 outside of the Beltway is between Lee Jackson (US 50) and the Beltway which are the areas with lane control. Even on the weekends, during the afternoon there's congestion both ways on this stretch of I-66. However, once you get west of US 50, it always clears up and traffic moves fine. I think adding an extra lane would help tremendously on this stretch, especially during non-peak times.
To echo what C. Patrick said a while back, the I-66/VA 28 interchange is terrible and I do hope to see improvements made there. However, it is better since the turn lanes were extended on VA 28 South. The traffic lights are synced up so once the light is green at Braddock/Walney, the waiting at the interstate to go East is minimized or eliminated. I've found equally as bad is the off ramp from I-66 East to VA 28 North. During the morning rush, it frequently backs up and drivers are always trying to cut in front as well. Also, people inattentive drivers can create problems with a sudden stop. It appears VDOT may be extending the lane for the off ramp based on the addition of a 5th light in the new lane control system which is not active yet. However, this could actually be worse, because unless an additional emergency lane is added it doesn't give room for drivers to swerve to avoid rear end collisions without going completely off the highway.