I recently used the USO at BOS and though it's not very large, it was comfortable, the rotating staff was friendly, and there were complimentary snacks and beverages. Fairly standard among USOs.

However, I was in a situation where I had to rebook a flight and my next flight didn't leave for another 10 hours, so the USO was very accommodating and let me leave my luggage there while I left the airport and walked around Boston for a few hours. Side note: the Silver Line "metro" (actually a hybrid bus/electric trolley) is free from the airport to South Station in Boston.