If you work around Union Square or are shopping with the hoardes, you don't have too many food options, other than random diners (Mel's, Lori's), Subway, Cheesecake Factory, and noisy food courts. Mocca transports you out of the chaos that is downtown SF and into a little European paradise, on a quiet, blocked-off little avenue with outdoor seating. The place is tiny but they have a great selection of fresh food - all of their gourmet sandwiches are amazing, the salads are good (I like the shrimp), and there are a variety of sides like pasta salad, fresh fruit, and rolls, not to mention a great selection of Italian sodas, coffees/teas, and alcoholic beverages. There is almost always a line out the door. I'd recommend it on a sunny weekday, so you can sit outside without battling the weekend shopping crowds. REMEMBER: they only take cash!!
Pros: ambience, food
Cons: long lines, cash only, a little overpriced