Final Update: I was able to move my appointment up twice yesterday, first to 7/15 and then today (6/15). If you're in a hurry, I highly recommend using the Chrome VisualPing extension. Just set it to monitor the "Month" calendar page using the "Browser" function.

Interview was in the Ronald Reagan building in D.C. The building is huge so I would give yourself a few extra minutes. Luckily, the folks who run you through the metal detector can point you in the right direction. There was no one ahead of me when I arrived and I was out in less than 15 minutes. From the sounds of it, someone called asking about Walk-Ins while I was there and the agent said they take them but only if there is no one with an appointment waiting. I would be confident in your chances if you show up and are courteous to them.

Only hiccup was my Passport and now my GE using my middle initial while my (Maryland) license lists my middle name. Anyone ever run into this issue? Only seems like it would be an issue on domestics.