In Slate, Jacob Leibenluft published an essay about coffee that gets at the problem a little bit deeper:
If your biggest concern is landfill waste, there's no question that a reusable cup is best. While it's technically possible to recycle a polystyrene cup or a paper cup, your office will be hard-pressed to find a way to do so...
But water use matters, too — especially if you're living in parts of the country, like South Carolina or California [or North Carolina], that have recently faced droughts.
Finally, there's the question of energy use and emissions. Here, the results get a little more complicated. Pound-for-pound, petroleum-based polystyrene is a pretty bad material—it takes twice as much energy to produce a gram of polystyrene as it does to produce the same quantity of ceramic. But you'll need at least 70 times as much energy to produce a ceramic mug as you will to manufacture a polystyrene cup, and probably even more to produce a stainless steel mug.