I took the shuttle bus from PEK to the stop just prior to Beijing West last week. I was staying at the Doubletree, and that stop was just across the street.

The bus runs every 30 min (I think at top and bottom of the hour), and cost was RMB24.

There's a ticket office just beside the bus stop at PEK. Then after you buy the ticket, you need to wait in line. Not everyone got on the bus for the run when I did.
They didn't use the undercarriage storage, but some guys loaded luggage into the first 2 rows of seats, which wasn't enough. Some people put their bags in the aisle of the bus.

I was toward the middle of boarding, and the only white person on the bus, sat about 2-3 rows from the back. No one sat next to me until nearly the end. I wasn't able to fit either my backpack or other carry bag under the seat, so had them both on my lap for the ~90 min trip.

Since you'd be getting off at the final stop, you won't need to worry about missing your stop (here's where having the maps on my phone going saved me), or having to walk over the luggage in the aisle to exit.

For my return trip, I splurged the 150 for a taxi