I love reusable take-away coffee cups, particularly the ones made from porcelain with a silicone lid. I keep one in the car, and one at the office. OK, you may not save money using the one that you keep in your car, but you or your employer will save money by not having to refill the supply of disposable cups all the time. You can also save money by filling it up before you leave home, instead of buying a coffee on the way.
Disposable cups create a lot of garbage, and waste a lot of resources and energy in their manufacture and distribution. There are also health concerns with some types of disposable cups, because they leak toxins into your hot drink. The biggest offender in this regard are polystyrene foam cups, as described in an article by Earth Resource Foundation.