Unless I'm heading for an A gate (or I'm tight on time), I invariably use the Dulles Diamond checkpoint on the baggage level and then head straight back for the D shuttle. I enjoy the view from the mobile lounge (like the previous poster), and I enjoy the hike - even going all the way to the low-number C gates.
The only advantage to taking the AeroTrain to C is the walk past the beautiful back-lit photos at the south end of the tunnel.
Note that the Dulles Diamond checkpoint closes at 6pm.