If you already own a Casio, or a Sharp, or another brand of graphing calculator, and you are familiar with using it, stick to it. But, if you are purchasing a graphing calculator, I would not recommend them. They allow you to do everything a graphing calculator can and much, much more. Some of Clark's computer labs have some of these CASs loaded and ready for your use. Derive is fairly inexpensive and easy to learn.
Maple is fairly comprehensive.
Commands are essentially a programming language. Some sections of Math 213 are being taught with Maple. Mathematica is the most complete of the CASs and the most expensive. The new version 4.0 is fairly user friendly, but still has a high learning curve. However, I find 3-D graphing in easier in Mathematica.