the functionality of synergy and keyclone are very different.

synergy enables you to use a single keyboard/mouse and move between the two desktops... working on each, individually, as though the keyboard and mouse were solely for that machine. for multi-boxers, this is very useful for running new instances on the various machines... but its a pain in the a** to mouse around to each machine hitting 'F1' for fireball.

keyclone, on the other hand, links applications together...enabling you to hit 'F1' within one application, and having that key simultaneously sent to each of the other applications keyclone is managing. it will also send those same keys to any other keyclones you may have linked in. theoretically, you could have a vast array of computers, each running keyclone and 1+ instances of wow, all connected... and a single 'F1' would launch a sh*tstorm on the enemy. (muahahaha)

this is obviously very useful for multi-boxers. (additional functionality is available, but you get the idea)

personally, i use both while multi-boxing. synergy for launching the apps, and keyclone for playing/synchronizing