I have used Synergy and Multibox. Each program has its draw backs from what I have seen so far.

Synergy -> A bit tough to configure. But once you have it going, it runs like a charm. Except for the crazy right-click to move bug, which I have yet to find a way to fix other than locking the mouse to that screen (scroll lock). However, that's damned annoying to do when I'm doing many things at once.

Multibox -> very easy to setup. However, after a bit of inactivity on the second PC, the whole thing will just stop working. And I have yet to get it to work again w/o having to restart both PC's.

*shrugs*