When you do a shot of tequila, you go salt, shot, lime.
When you do Schwartz sandos, you go bite, mustard, pickle. (hint: Cut up the cornichon in bite size pieces and squeeze a bit of mustard onto your plate.) Mustard adds some tang and pickle's acidity goes ha-YAH! Like Beatrix Kiddo slicing O-Ren Ishi's head.
This place got me into Jewish Delis which made me find Wise Sons in SF. I'm a Rookie to the Jewish deli scene so I still need to give Katz a chance to Pai Mei... Or not.