While Scott's concern about the added cost of cables for the "dongle" type KVM are valid, they do bring a couple of advantages. We use Advocent KVMs and as they use a dongle that takes the VGA&USB&PS2 connectors and converts to Cat5 type cable, I can make the Cat5 cable the length I need. Cat5 cables probably fit into your cable management system on the racks better than the bulk KVM cable. Some "Non-dongle" type KVMs still require special cables, which brings in the same cost considerations that Scott mentioned.
Another feature to look at is KVM over IP. Basically you can assign an IP address to the KVM, and get to it via a web-browser. This means that if you want to reboot a server, and the boot hangs for some reason, the IP KVM can save a trip to the server-room. Combined with VPN, and I can reboot a server and change BIOS settings from home.
A word of advice, practice using the KVM BEFORE you need it. You don't want to be learning which button swaps servers, or any quirks in the interface while troubleshooting a down server!