Silver Line Express Bus

Free wifi, $ 5. Reston to IAD and runs about 15 – 20 minutes.
It’s a company contracted by IAD. I used them once and liked the experience. The busses were decent and the drivers were polite and drove pretty well. It was a smooth ride and is quite a reliable service.

This is the cheapest and easiest way to get from the end of the Silver line to the airport. However, expect no signage explaining how it works, no answers from the driver if you ask, and no announcement when you reach the airport.
You have to pay at the airport end and they only take cash and credit cards — no Metro cards or tickets. They also drop you off at arrivals not departures, so you will have to head upstairs to check in for your flight.
This is a necessary service but has huge room for improvement. I’m thinking I would have been better off getting off the Metro earlier and taking the 5A to IAD.

This service extends the Silver line from Wiehle-​Reston East to Dulles International Airport. From the metro station the bus signs will lead you straight to the bus station. It’s cost you 5 dollars, to be paid at arrival. Calculate some queuing into your schedule when travelling. Ohh, it was cash only when I arrived. Don’t know if the terminal was down, or they are a cash only business.
They offer free Wifi on the bus, and I was surprisingly able to stream good quality Netflix. I don’t know why I did load that app though, as the bus only took 10 minutes to the airport.
Buses are fairly new and clean, and have designated areas for stowing luggage. I arrived just outside the terminal building, ready to check in.

Necessary service and good addition to the current public transportation to Dulles. Picks up at end of Silver Line (Reston stop) every 15 minutes during weekday business hours (check schedule before you go, it’s not currently in Google Maps but is in WMTA Trip Planner). Ride takes about 13 minutes but I imagine it might vary in traffic.
Major downside: they don’t take smartrip and only accept payment once you arrive at Dulles. So if you don’t have $ 5 cash handy you might be waiting in a long line while you’re already stressed about making your flight, be warned!