There are two ways that USB KVM's tend to work; one is to act as a USB hub, the other is to act as a native USB keyboard and mouse. I will try and detail the pro's and con's of each below so that you can decide which is best for you.

KVM as a USB Hub
These are the slowest but cheapest of the two; switch over usually takes a few seconds on a good unit, but sometimes takes as long as 10 and in the case of multiboxing your switch over time is critical. This is because windows needs time to perform the handshake necessary to get the hub online and your USB devices connected again (not the same as installing drivers). The only real benefit is that keyboards like the Logitech G series will remain fully functional (provided that software is installed on each system), and that a software programmable mouse will also function provided that its software is installed and configured on each system also (some Razor and SteelSeries gaming mice store profile information on internal memory so this is not an issue with them). Apart from the switch over speed one other major draw back is that you can’t boot all your systems up at the same time; you need to be switched over to each system as it boots in order for it to see your mouse and keyboard. These can often be found for less then $50.

KVM as native USB Keyboard and Mouse
These offer an almost instantaneous switch over in most cases but can cost considerably more; a reasonable unit would cost upwards of $100 and could set you back several hundred. These offer a dedicated port for your USB keyboard and mouse and fool the systems into thinking they are always connected. Quite often these will include a USB hub as well for convenience. The only major drawback here is that the above mentioned keyboards will not be fully functional due to the way these KVM's work to give you quick switch over speeds. There is no pass through between the active system and USB device and in the case of wireless mice you will not have battery level indication if provided by the mouse software. These on the other hand do allow all your systems to boot at the same time without the need to switch between each.

I have had quite a bit of experience with several brands of USB KVM for multiboxing, and in the end I have chosen to use PS/2 :P Even after spending close to $400 on a USB KVM I still had occasional issues where the system I switched to had lost the keyboard connection requiring the USB cable to be unplugged. PS/2 is completely reliable, fast and has more configuration options over USB. PS/2 also has the advantage of still being the de facto input for most servers and racks so there’s plenty of server grade equipment out there (I personally use Vetra products for their reliability and range). This is just my recommendation but I do understand how hard it can be to part with your favorite USB keyboard for an old PS/2 relic that looks like it belongs in a museum; though if your willing to spend the money there’s some wonderful gaming grade ones out there and paired with the PS/2 X-Keys you’ll get back your lost macro buttons.

I hope this helps you in your decisions, good luck with whatever you chose
-Aldercy