I live around 43rd and Pine, which is about 5 blocks west of campus, and I think the area is great. Penn stations security guards at almost every intersection between 39th and 44th, and from Baltimore (on the south side) to.....at least Walnut, although I'm not sure what the northern boundary actually is (someone please correct me here!). They walk around and hang out on the corners from dusk until dawn 7 days a week, so there should always be one within shouting distance.
🙂 In addition to the shuttle Cyrille's apartment runs, Penn has a thing called PennTransit, which has shuttles that run along several routes until pretty late at night. I've never taken it, so I can't give you details, but you can read about it on Penn's transportation page.
I have a moderately sized 1BR in one of those 5-unit Victorian-style duplexes, which I share with my boyfriend. We pay $790 a month. Within easy walking distance (< 5 minutes) is a Vietnamese restaurant, pizza place, CVS, grocery store, a couple bars, a couple little cafes, a dry cleaner, a laundromat.......it's really convenient. I ended up renting from a guy who only owns one building in the area, BUT while I was looking for apartments I came across a place called ARCCO-Realty World, run by Alex Chamberlin, and they have a TON of buildings in the area, many of which are pet-friendly. I looked at several of their apartments and ended up liking mine a little better, but Alex was really nice, and very accommodating, and seems to like renting to vet students, so if anyone is looking for a place I recommend talking to him.
Oh, I almost forgot -- as has been mentioned already, the neighborhoods change pretty dramatically within short distances, so you could be in a great neighborhood on one block and then in a terrible one two blocks later. Penn compiles crime data and makes density maps available, which you can sort on a block-by-block basis, which is really nice. The URL is
http://cml.upenn.edu/crimebase/
Good luck, everyone!