I live between PHL and BWI and fly mostly US and UA. BWI tends to have some VERY reasonable fares on UA. I fly mostly transcon - so the savings can be considerable. I was able to get a UA BWI-SEA 2 weeks out for $120 RT after applying a $100 off voucher (I got when a TED flight left me stranded in LAS).

$220 RT for a transcon - all in - is pretty nice.