I've been to all the big american cities and Philadelphia is the sketchiest of them all imo just ahead of LA. Just doesn't feel safe even in the city centre, crackheads as far as the eye can see, dodgiest metro system I have ever been on (people openly injecting smack etc). We were there to see Springsteen so we did that, the Rocky Steps and liberty bell but there didn't seem to be much else to do after that. I was with the blues in NYC and Chicago in the summer and both are miles better than Philly especially Chicago which was absolutely spot on.