Is your ticket restricted to a certain flight? If so, I've found that they are very strict regarding time-restricted tickets on Shuttle. That said, if a plane will go out with empty seats, you might get a gate agent who will move you to the earlier flight, so it can't hurt to ask! Even if you don't make, the US Airways Shuttle terminals in all three airports (BOS, LGA, DCA) are quite nice. If I recall correctly, BOS has rocking chairs that overlook the apron. If you're a US Airways Club member, there is a very nice in club in all three airports. But it's rather far from the Shuttle gates at LGA, decently far at BOS, and a tad far at DCA.