Update: US Airways lounge is now AA lounge
I was just at the BOS Terminal B a month ago and AC still has a deal going with the AA lounge (formerly US Airways lounge).
I found this lounge much nicer than United! They had some hot soup and some nice snacks (as well as self-serve bar).
You will still have to do security twice, but they had pre-check for the main terminal area. Then they have an expedited process for AC departures area where you still have to pull your liquids / laptop out but can keep shoes on.