I flew back from Boston at the beginning of June. The check-in agents told me that AC has an "arrangement" with US Airways to use their lounge, which is very close to security. While it means two trips through security, it was worthwhile, and the walk from the US Airways lounge to the AC security line was less than 5 minutes (including taking the down escalator to the baggage claim area and the up escalator back to the departures level).
The AC security line only took a couple of minutes, and as noted above, led to a small waiting area that was overcrowded, with a very basic coffee shop/food counter and no room to move.
I was very pleased with the recommendation to use the US Airways lounge - the staff there did not question the AC boarding card, and the lounge was spacious (not very busy at 16:00 on a Friday afternoon).