I've been using these at home for years now, and there's quite a range.  You
can easily pay over a thousand dollars for one, or you can get one for less
than $30.

The mechanical rotary switch type were ok for DOS and VGA or less monitors,
and they can be used even up through fairly current equipment.  There are
three drawbacks:  Weak connections due to oxidized contacts make funny
colors and odd mouse/keyboard errors; uneven contact during switching can do
weird things to monitor sync, and there's  a chance you can short two
connections together, which may very well fry your motherboard (done twice).
If you go this route, consider getting more connections than you need and
leaving one blank connection between each "live" connection for cleaner
switching.

Then there are the passive electronic ones.  These offer clean switching and
eliminate a lot of the headaches of the mechanical switch.  The only problem
with these are when you have software that checks for the mouse or the
keyboard.  For instance, if NT reboots and it's not the selected input, it
will fail to detect your mouse and there's no way to load it.  Mostly Linux
doesn't have a problem with this, if you're in text mode you just start GPM
manually.

The fancy ones actually emulate a keyboard, mouse, and P&P monitor back to
the input/workstations, so even if you boot without being selected, you
appear to have a live mouse.

Price wise, you can get four station passive electronic switches for under
$100, and you can get decent four station switches with emulation starting
at about $120.  Shop around, watch the specs, and you should do fine.

I've seen ads in catalogs that implied that the unit could connect to both a
Macintosh and a Wintel machine - that's some feat of emulation/translation.
I've never actually tried one, but they may be out there.  I've also never
seen a USB switch - that sounds like something that would be very hard to
do.