Along the same lines: I launched a social calendar app with my roommate for students at UMichigan and to initially gain users, we bought $55 worth of pizzas and to get a slice, all students had to do was download our app and register as a user--we setup in one of the busiest halls on campus and needless to say, the pizza ran out pretty quickly.

Looking back, it was great for the numbers and a was a good marketing strategy, but we didn't gain much else from it. I wish we had setup a demo or something of the sort so that we could have also received feedback. Asking vital questions about the product should be geared toward customers, not as much founders, and we learned that the hard way.