Input Director ROCKS. It is easier to use and has seamless integration. Crazy GOOD. I mean it is beyond best
I have used both Synergy and Input Director. ID is slicker and MUCH easier to set up than Synergy. Once the configuration for Synergy is complete, it's great, too. The screen edge visual in ID is somewhat distracting, but that can be eliminated. The major difference I notice is that ID seems to be a tad jerky as the two PCs I use are not on the same subnet. The laptop is VPN/wireless and and the desktop is on a wired network. Synergy shows some jerkiness as well, but not as pronounced. Ease of use goes to ID, but Synergy has better throughput. Switching to a wireless network that did not require me to use VPN did not significantly improve Synergy or ID. The latency was still apparent. Since most folks will not use two PCs connected in the same fashion, either program should do the job well.
I plan to continue to use Synergy since at home I have a Mac, a PC (or two) and Linux. But although Synergy can handle three screens, no way am I going to put three displays on my desk.