I use Kavoom KM, Keyclone, and Twobox as well. Kavoom practically sets itself up for you, all you really need to do is drag and drop which computers screen goes where, and type in the name of the other computer that you are using, during the setup. It will even install itself on the other computer.

The only issue I've had with Kavoom and Keyclone is that if i minimize Keyclone from anything other than the toolbar, my mouse will get stuck on the monitor I minimized Keyclone on. I've just learned to minimize it from the toolbar and I don't have to worry about that issue. Everything else works fine. Kavoom is $40 dollars for the first two computers, and $20 dollars for each additional computer. The link to Kavoom is in the sticky.