Considering that the majority of FPs are scheduled within 1-4 weeks of receiving FL letter, doing a walk in doesn't speed up process dramatically.
Well, if you have time to do walk-in FP, do it. Early FP would ensure your app will not be delayed in case you have last minute schedule conflict.
I tried to do early FP in Atlanta (Collier Rd) and did it at the worst possible moment. Apparently Tuesdays are always busy (ASC is closed on Monday). I went for my scheduled FP which was on Wednesday and there was hardly anyone inside.
My letter mentions that in case I miss my appointment, I can come by any following wednesday. It seems wednesdays are the slowest days for ASC
I don't see much reason to do walkins nowadays.