I have developed recurring cravings now for their Big John sandwich (salty roast beef, yummy bun, great mayo... YUM), so I frequent their shop on Grant Street when I have an audition in the city or an appointment after work.
If you'd like to go when absolutely no other customers are there, 6pm is a pretty good time. They're open late (9pm, I believe) even though their neighbors often close early, given that the business crowd has gone home to families.
One curious thing about the chain (not particularly this one location) is that they don't offer a 'toasted' option. I'm aware it's because their gimmick is "fast", but I'd salvage 90 more seconds of my time to have a warm sandwich. I couldn't believe they survived competition when they handed me a cold-cut sandwich... though it is a DARN good cold-cut sandwich.