Unless you count a three-month internship in college, I've never lived in New York City. But, between friends and work, I've managed to visit every couple years or so and I've nearly always picked up an unlimited-ride Metrocard for my week in town. Turns out, choosing to do so is an excellent example of Maggie not being super great at math. Michael Moyer has plotted out the numbers on unlimited-ride Metrocards.
He says the purchase only makes sense if you're riding a lot—averaging 14 rides a week for the 7-Day-Pass or 12 rides a week for the 30-Day-Pass. Any less and you're actually better off paying a la carte.