My guilty pleasure. Seeing as it's open till 3 AM, it's perfect for late-night grubbing when all the bars are closed. This is where I managed to snag a quesadilla 4x the size of my head -- the quesadilla para dos is actually more like quesadilla para ocho! FYI, their green sauce is amazing.
This is a great free place to go for an hour. You see some history of San Francisco, get to see the machines that make the system work, and after that you are in Chinatown, so can get some awesome food nearby.