KC has an option, to broadcast a click from the main window to each of the slave windows in turn.
This broadcast is not quick or instant, basically taking 2-3 seconds for five clients, during which you cannot do other stuff.

If mouse broadcasting is important, you want either HKN or IS Boxer.
If that is not too important, then KC is very good for key replication.

IS Boxer has a bit of a learning curve.
The wizard gets you a functional set up, very quickly.
Adapting the basic setting to how you want it to work, has the learning curve.
It isn't that complex; I've managed to learn to use it.
And there is a lot of support, in the form of more experienced users, the www.isboxer.com forums and the IRC (linked from www.isboxer.com).
Once you understand the basics on how the software works, its much easier to change/adapt then a script based language.