Philadelphia actually has fairly decent mass transit when it comes down to it.
The subway coverage isn't fantastic, and it needs expansion down to the Navy Yard, and possibly a new Roosevelt Blvd line. The real gem is the regional rail coverage. The map below has both the subway/el coverage along with the regional rail coverage:
The other nice thing about Philadelphia is that in the city core you can get a bus on pretty much every single corner of every street.
This PDF shows the coverage, with bus routes marked in red.
The subway is kind of dirty but it's cheap and quick and runs frequently. When the subway closes for the night they run buses along its route for late night commuting. A monthly pass for the city itself is around $80, and you can get an unlimited pass that covers all regional rail stops (so basically coverage to all suburbs) for $200 a month.
Another big perk is the PATCO line, which will take riders various town in NJ, and is linked to NJ transit. Additionally, AMTRAK services the NE corridor and you can catch AMTRAK trains from 30th street station. It runs from DC to Boston, and includes the high speed Acela line.
One thing not many people know is that you can take NJ transit trains from Philadelphia to NYC for $15.50 one way. It's amazingly cheap, but the ride is fairly long at around 3 hours. The other alternative is to take AMTRAK which is something like $70 one way, but that trip is half as long.