Should I choose Synergy?

Synergy: Multiple computers and multiple screens

Synergy is best used when you have multiple computers on your desk, each with their own screen. Synergy allows you to only use one keyboard and one mouse with all of the computers. This is what Synergy was designed to do, and if this isn't your goal, you should use a different tool.

Hardware KVM: Multiple computers, but only one screen

If you have multiple computers, but only one screen, you probably want to use a KVM. There has been some work done with Synergy to help turn it into a "soft KVM" (see issue #304, and possibly #360), however, this is really outside of scope.

VNC/RDP: Remote computer access

If you want to work with remote computers (ie. not physically in the same room as your desk), you may want to use something like VNC. You may also be interested in technologies like NoMachine NX which is more efficient than VNC but only works on Linux.