From what I've seen, the inner city is nice, but some outer areas look really bad.
However, the most impressive thing, that I have never seen before in my life, was their citywide free WiFi network. Since I had my PDA with me, I did some 'wardriving/warwalking' and their WiFi network was pretty much all over. Of course, there was no continous coverage, but the downtown was pretty well covered so you didn't have to go very far to get near a hotspot.
I knew that Philadelphia was planing to get a citywide WiFi betwork, but I was under the impression that their project was stopped by various ISPs that were bitching about it. So I was pretty surprised when I went to the visitor's center, turned on my PDA, and found an open network called something like "phillywifi". However, skype didn't work, nor did any programs such as Mediaplayer with internet audio streams.
So I fired up the web browser, and tried to go to bbc.co.uk but instead I was redirected to a local web page, which said that in order to access the network, to log in with the username: liberty and password: bell
So I did, and got a message that now I have access to the Internet. Everything worked great! I even called my mother in Romania from Skype, so I had to pay like 8 euro cents/minute. If I would have called her from my cellphone, it would have been 2 USD/minute.
They really should do this in other cities as well, it's an extremely useful thing to have in a city.