I'm a paid user of synergy and despite not necessarily loving the attitude and the direction of the company behind it, I find the software invaluable. Not just for the space saved on my desk, but also for copy-pasting across different environments (though it breaks on rich text - I have to wash things through notepad at times), sometimes I even abuse it uuencoding small files and moving them across. I use it on 2 different mixes of Windows, FreeBSD, MacOS and Linux(Pi).

Another big win is the mental order it ingrains in me - e.g. this screen is for sensitive browsing on a sandboxed machine, this is for shells and corporate intranet, that is the gaming box etc.

Also - its "follow focus" (really follow machine) behavior is the perfect sweet spot between alt-tabbing and true follow focus: if I move the mouse to the next screen, I'm focusing an app in that machine. Move back, my inputs go to the last focused app on the previous system. Can't explain this easily in words but it's so good - better than 2 monitors hooked to the same box.

I'll keep a close eye on barrier. I'm happy to see its open source core remain available and maintained.