Obviously I still use FreeBSD on the desktop; with the packages from area51 I have a full and modern KDE Plasma environment. We (as in, the KDE-FreeBSD team) are still wrestling with getting the full Plasma 5 into the official ports tree (stalled, as so often it has been, on concerns of backwards compatibility), but things like CMake 3.10.1 and Qt 5.9 are sliding into place. Slowly, like brontosauruses driving a '57 Cadillac.
In the meantime, I do most of my Calamares development work -- it is a Linux installer, after all -- in VMs with some Linux distro installed. Invariably -- and especially when working on tools that do the most terrible things to the disks attached to a system -- I totally break the system, the VM no longer starts at all, and my development environment is interrupted for a bit.
That's always a good moment to switch distro's .. since I'm going to spend an hour or so re-invigorating the VM anyway, reminding myself that this time I'll make a clone and keep snapshots and whatnot, I may as well see what others use Calamares for. I've left a dusty and broken KDE Neon and a Manjaro behind me, and today I'm starting on Kaos.
For the very reason that Calamares can break stuff, and because new Calamares versions need to be tested on (older) live ISO images, there's a deploycala Python3 script that helps install all the (development) dependencies needed. This is a bit of a drag, since it's tied to a whole bunch of distro's specific package names, but it gets the job done. I update it whenever I hit a new distro.
Kaos is a KDE-leading-edge distro, and daaannnggg is it ever slick in the first 10 minutes of use. The customization of Calamares is some of the nicest I've seen. The release-notes page could use some work (on my part) .. if only because I could read that stuff during unsquashfs, instead of before it. First-time startup with Kaptan is a hardly-intrusive way of configuring a bunch of separate things that would require a careful trip through systemsettings.
Look for the next few Calamares releases (in particular 3.2) to emerge from the Kaos, then.