I was in Charlotte for a week on business and a colleague recommended 521 as "the best BBQ you've ever tasted." He's from NJ, so maybe that skewed things, but 521 was good. Not great.
I got the ribs and received the thick end of the rack, plus two ribs from the thin end. The ribs from the thin end got a 9.5/10 rating. Well smoked, pull-apart and with a delicious crunchy glaze. The ribs from the thick end were cooked, but not to the point of BBQ doneness. They were tough to the point that I had to stop at CVS and buy dental floss.
Eventually I gave up on them, and just ate the excellent hush puppies and mac & cheese. Good sweet tea and friendly enough service. Clearly a joint for locals but it was friendly.
Nice place overall. Maybe I just got a bad plate. On my next trip to Charlotte I may go back based upon those two excellent ribs.