Just used the shuttle. I arrived at C and needed to transfer to B, and the walk from C to B (US side) is so insanely long, so I thought I would kill two birds with one stone and do a little research for everyone else on FlyerTalk and also shorten my walking time to B (A-B feels like half the distance as C-B.)
The bus departs from a small, almost unmarked area near the CSC just before the UC entrance. There is a small set of stanchions against the wall; one of the stanchions has a small placard above it that says, "Wait here for the shuttle to terminal A." I missed the sign at first, so I ducked into the club and asked the attendant if there was a shuttle to terminal A. She told me where it was and then said she would call the driver. Good thing she did, because there was no one at the CSC, and it didn't look like anyone was paying attention to the shuttle waiting area.
A couple minutes later, a guy walked up and opened the door for me and led me downstairs to board the shuttle. Note that he did ask me for a boarding pass; I stammered for a second and told him I only have a boarding pass for terminal B, but I was meeting a friend in the United Club in terminal A. He said that's fine; he just needed to see a boarding pass. I showed him my pass for the B concourse, and he whisked me through with no problems. The shuttle was already waiting at the door downstairs, so it definitely seems like it's on-demand. It's pretty quick, in any case, since it doesn't seem to be heavily used (given that probably not many people connect in BOS) and is a short ride anyway.
A few minutes later, we arrived at terminal A down at the end of the long lower hallway for the RJs (gates A9-A12). Another attendant then led me upstairs to the main part of A, where I exited security (after making sure he wasn't watching
) and walked to B, arriving 5 minutes later. Total time: probably 9 minutes versus what feels like a 15-minute walk from C.
I didn't notice where you are supposed to wait in A for the shuttle, but I'd assume its somewhere near the A9-A12 gate area.