I've been using it to control 4 PCs (3 laptops, 1 Desktop), and is a great alternative to Remote Desktop; particularly if you need screen real estate testing network(ed) applications. You don't end up sacrificing the real estate on the 'main' PC with remote desktop, you don't need a vast array of video cards to extend your working area, and you can put those old laptops/PCs to at least some use.
Essentially, it sends keyboard and mouse commands over the wire (I believe there's at least one 'open source' application which does the same thing) to the other PCs.
My mouse is setup with the 4th/5th buttons to move to each of the other computers. Within a few minutes of practice, the whole array of computers are being controlled simultaneously from one keyboard and mouse.
Got to tell you, folks, I heartily recommend it!