The KVM switch that you choose has to be capable of multi platform support if you want to host machines such as MAC O/S, and anything other than a PC. This is because the KVM switch emulates the keyboard and mouse signal to the operating system so it has to know how to communicate. With USB this is alot easier because you do not have to worry about the computer recognizing anything but a USB Device.
Most of the time standard installations are going to just have PC's using standard hardware, but in some more sophisticated setups you will find a mixture of many different platforms all hoping to be accessed by the same KVM switch.
Make sure before you buy if you need support for MAC or SUN systems, that your switch has that.