I use the Belkin Omniview F1DS102T PS2/USB kvm switch. It allows me to share a keyboard and a CRT monitor btwn my mac and my PC. The PC is connected via PS/2 and the Mac via USB. Each machine has its own mouse (as they don't take up much space) as well as its own lcd panel so that I can use both machines at once.
The image quality is quite good, with both machines at 1600x1200. There is some leakage or interference visible when both machines are set to the same refresh rate, so I run one @ 75 hz and the other at 85 hz, which fixes it. The use of premium video cables is recommended.
The only other issue I have is that I don't seem able to map the keys of my Microsoft Multimedia Natural keyboard through the kvm (command and option) properly, so I keep my original apple keyboard handy for use with Illustrator and Photoshop.