The Fairfax Connector bus does take Smartrip cards.

The problem from my standpoint is that the stop for the Silver Line Express is nowhere near the stop for the Fairfax Connector (and the 5A) so you have to commit to one without knowing which will come sooner.

I took the Silver Line Express yesterday and there are a few annoyances. The luggage racks have inadequate space and were completely full before the bus was half-full. The seating is bench seats, rather than the comfy padded ones. The bus stop at Reston is in the parking garage, connected to the trains by a pedestrian bridge. But the first part of that pedestrian bridge has no canopy, so will be unpleasant at best in bad weather (and, inevitably, will never be shoveled / deiced in winter).

For those of us who live on the western stretches of the Orange Line, it takes at least 15-20 minutes longer to get home from IAD now. (And I was lucky yesterday and had only a 6 minute wait for the Vienna train.)