I own such a switch. Bought a cheap switch (I believe the brand was "Kobiyashu") at a computer sale and it works really well. As far as I can tell the image quality is equal compared to when my screen is directly connected to any of the systems. Switching between different systems can be done by either a set of hotkeys or the button on the KVM. I'm using this 4-port KVM for connecting my server, secundary desktop, laptop and random others pcs to one screen/keyboard/mouse.
They have all kinds of nice KVM switches these days btw; some feature audio, USB and even firewire or lpt. Proffessional KVM switches are accessable through telnet, but I guess that's not what you intend to buy