Maybe I will end up loving Philly, but right now my ambivalence is about the fact that every picture of the city I see that's not either Penn-related or major-historical-sight related seems kind of run down and depressed (in an economical sense, but also I think that can lead to a certain depressing kind of aura). I asked my friend living there to go around town and take a bunch of pictures of various places: the grass in the parks seems to be brown rather than lush, the buildings seem to be needing a lot of touch-ups, and the streets seem kind of empty.
I'm used to living in a very crowded and intense city and Philly (again, just from pictures) seems to feel a bit empty, almost lonely. But everybody who's actually spent time there says that it's not that bad or even that it's great, as lovebutton did. So I'm looking forward to being wrong!
I'm not going to Penn (well, I'm still on the waitlist), but I lurk a bit here anyway out of nostalgia...
I was an undergrad at Penn. Philadelphia really is a fantastic, vibrant city. There are some depressed areas, but anywhere that you'll have reason to go to is great. I don't know where you saw pictures of, but everywhere from University City east to the Delaware River is just as crowded and "city-like" as any other major city. And it's only getting better, with the new development in Center City. Plus, Penn itself will be undergoing massive expansions over the next decade or two (they just purchased land totaling I think 50% of the current campus size), some of the early stages of which you might benefit from. And a proposed footbridge over the Schuylkill River (which I think is supposed to happen within the next couple of years) connecting the campus directly to Center City will definitely help.
Basically, if you're coming from New York, you might find Philadelphia a bit less "intense." But from anywhere else, it's the same or better. I'd say at worst, comparable to Boston or Washington.
P.S. The title of this thread annoys me every time I see it. It's "Penn," not "UPenn!" You only say "UPenn" when speaking to someone who you predict won't be able to tell it apart from Penn State!