Thanks for your input. I ended up booking the appointment on June 6 right when the center opens (8 am) to try to avoid long lines.I would make an appointment. USCIS centers only have two weeks at a time for booking an appointment on their website. I went to the SLC one, and had scheduled the appointment two weeks out from when I received my biometrics request. And when I got there the day of the appointment the officers that check you in through the metal detector ask to see the appointment sheet as well as your biometric request sheet, so I don't think they let you in if you don't have an appointment.
The SLC USCIS was busy. My appointment was for 10am, but I sat there for a good 30-40 minutes before they got to me. Hope this helps.
You made an appointment, keep it. But if you are near a USCIS you can def try to walk in and see if you can get in earlier, but being they ask for proof of biometrics request and appointment, I don't think you'll have much luck. It's worth a shot though!