Pizza Mercato serves passable pizza for cheap. You can get the 2-slice+a drink deal, but beware, the drink is tiny. The sandwiches are okay, and service is fast, but for my money, I'd rather walk across the park for The Pizzeria on MacDougal (halfway to Bleeker St.) They don't have the starving-student deal, but the slices are still cheap, and the pizza is just better.
Mercato's is okay, but if you're doing the shopping stroll down Broadway, do yourself a favor and head for The Pizzeria. More seating there, less of a college-student crush at peak times, too.
Pros: fast, cheap, just a block fm B'way
Cons: crowded, crappy seating