It is kind of hard to rate a place like the Mellon Center, as it is pretty much just a building with a few shops in it, but hey, five stars, why not? After all, the building is rather convenient to pass through when it is raining, or if you feel like chilling out in a not often used, clean, indoor public space.
Underneath the city of Philadelphia is an extensive network of tunnels, which generally connect modes of transportation, and a lot of the Penn Center buildings (which were once upon a time owned by the Pennsylvania Railroad). The Mellon Center at 18th and Market is one far extreme in this network of tunnels, allowing you to traverse underground to City Hall and beyond, which can be rather convenient in the winter and during a rainstorm. (Note that the connecting concourse to Suburban Station seems to close at about 7pm on weekday evenings and may be closed on weekends).
The shops themselves are convenient - there's a gym, a cafe or two, a UPS store - but my favorite part of the building is the public space with tables and chairs that is nice for having lunch or hanging out for a bit while passing through. I'm not sure if there is any specific name for it, but I've heard to it referred to as the "winter garden" or the "arboretum" - but in some ways it resembles a greenhouse, with tons of windows to let light in, and a small fountain in the back. I've met my husband here several times for lunch, and it is a nice, often quiet spot, rarely full. read more