My pet peeve with the GUI is the scarce information once you establish connection with your slave machines.
After connection I think the GUI should either start a wizard for placement of the slave or otherwise indicate strongly in the GUI where monitor placement is AND that it is the next step.
I almost forgot: Is there an option to have your slave machines to just one side of your main machine monitor? I couldn't figure it out with the GUI as is anyway.
I have installed Multiplicity twice and both times I connected and expected... something ...and then tried to connect again (even though there was nothing wrong with the connection).
Could an "advanced" option be to not copy a file to another machine just because you open a folder? I have been copying large ISOs (yes, I use VMWare in conjunction with Multiplicity) and every time I have copied an ISO to a machine that copy "hangs around" and when I go to one of my other slave machines and open a folder I get a new copy of the ISO file on that machine.
Otherwise, as I've said - it just works and works and is a real joy to use.