On the subject of the A to C shuttle: in my limited experience it appears to be a complete... well, the board will censor me if I use the word I think is appropriate, but let's just say it's a mess.
When I got there, there was a huge mass of people standing by the door -- maybe sixty or so. Not in any kind of organized line, just a huge mass. I waited at the back of the mass for about five minutes, then the extremely detached-looking airport employee at the front let a handful of people through the door. Someone expressed surprise at the small number of people going through, to which she droned "It's a twelve-person bus".
I groaned loudly and walked off through security (apparently leading a small exodus in the process), caught the train to terminal C, got back in the premium security line, waited around for a few minutes, figured out that the non-premium security line was moving a lot faster, and switched to that one (I kept pace and noticed that I got through a lot faster than the guy who had been ahead of me in the premium line).
Anyway, if I'd actually hung around to catch the A-C interterminal bus I estimate I'd probably still be there.
On the upside, once I got through to Terminal C I found that the United Club just to the left was perfectly fine and had good seating options and free Sam Adams to go with the usual extremely limited food selection. Couldn't complain.
Have other people had better experiences? Is it always a twelve-seater bus? Is there always a huge mass of people?