Be aware that they come in various configurations. Some have audio support while some do not, & some have USB or PS2 support (though adapters are available). If you use a TFT monitor through its DVI interface then you'll need one that caters for it.
Hence make sure to get one that best suits your needs. G
They work well, even fairly cheap ones. The cheapest may give slight video impairment on high-end monitors, but I haven't suffered from this myself.
The only thing that I have found irritating is I have one KVM setup that uses a double press of the CTRL key to switch..... Amazing how often a CTRL C (copy) followed by a CTRL V (paste) sends you scooting off to the other machine.
Some use "odder" key combinations, others (I like) have a programmable key... so I have ALT SHIFT on one, and that causes me no problems.