Philadelphia! I lived there for six years, just moved away a week ago. Here's my input:
Basics - As a city, it's a mix of dense downtown and urban sprawl. The 'Center City' area, located between the Delaware and Schuylkill rivers and between Spring Garden street on the north and South Street on the south, is where a majority of attractions are located, including arts and entertainment, restaurants, and bars. The rest of the city falls into the following categories: South Philly (south of South St.), West Philly (west of the Schuylkill river), North Philly (north of Spring Garden St.), and Northeast Philadelphia, which is the weird little arm of the city that follows the Delaware River north and east away from Center City.
Transportation: SEPTA, the Southeastern Pennsylvania Transportation Authority, is usually adequate. They operate city busses, regional commuter rail, and the subway lines. Busses are your typical urban bus service - inconvenient schedules, often crowded, and not exactly sparkling clean. However, they usually get you where you're going. Their coverage of the city is pretty good. The subway lines are similar, except for coverage. There are only two lines - the Broad Street line runs north-south along Broad St, and the Market-Frankford line runs mostly east-west along Market St., with a turn to the north at the river. The Regional Rail lines offer service out to the suburbs, from three stations in Center City and West Philly. I never owned a car while I lived there, so I can't really comment on driving. Parking lots abound, though, and there's on-street metered parking in most of the city. There are also neighborhood parking permits, which allow you to park on the street in your neighborhood. I guess without a permit, you get ticketed.
As far as cost of living goes, a good guideline is that the closer to center city, the more expensive. The (IMO) nicest neighborhoods to live are the Rittenhouse Square area and Queen Village/Bella Vista. Both are very beautiful, very safe, and very expensive. The 'up-and-coming' residential area is a neighborhood called Fishtown, in North Philly, about a 10-minute drive to Center City. It's undergoing a large gentrification at the moment, but is still affordable.
My strongest recommendation of a neighborhood goes to South Philly. That's where I lived for the past 17 months. It's a geographically large area, heavily Irish-Italian-union, but with many ethnic sub-neighborhoods - Mexican, Korean, Vietnamese, etc. Residential buildings are almost entirely row homes, but there are a few apartment buildings scattered about. Major retail is confined to two shopping centers, pretty easily accessible by car and bus.
I hope this helps you in your decision, and I'd be happy to answer any specific questions you might have.
-John
But life's not dull anyway which is always good!