I've also mentioned this several times.
That said I actually know some people who use *BSD for everyday desktop usage. It's also highly popular in universities and among researchers. I don't have that much experience with alternative UNIX, but FreeBSD is by far the archetypal BSD distribution, the most maintained and generally longest running. Fact is, Microsoft and Apple have directly 'borrowed' code from it for things such as the TCP/IP stack.
OpenBSD is generally considered to be the most secure with its built-in pf firewall, crypto extensions, process isolation and highly strict code auditing. This has hold true for the most part, although the base system is pretty minimal. Vulnerable userland components that you install later don't count. However, there were also allegations at some point of a backdoor in the IPsec stack for usage by the FBI. This was audited and never confirmed, but personally I don't trust it much.
NetBSD was the first open-source BSD after 386BSD and is primarily used for servers, routers and embedded systems. This is because it makes use of many lesser known hardware features and it is by far the most portable of all BSD distributions, compatible on a boatload of processor architectures like no other.
Then there have been a whole bunch of later distros like PC-BSD and GhostBSD, which are meant primarily for desktop usage and have all sorts of preconfigurations to make it easier for the user. DragonFly BSD is a pretty famed fork of FreeBSD from 2005 that became pretty hyped for its custom 64-bit file system made from B-trees called HAMMER. Its performance is still not up to par last I checked, but I can't speak for certain.
BSD is well-known for the way it handles the initialization daemon (the BSD-style initscripts), where it keeps startup scripts in /etc/rc.d/, plus a small number of configs. This is in contrast to most mainstream GNU/Linux distributions, which use the System V approach with /etc/init.d/
The BSD distributions are also famed for their package management system, which is generally referred to as the ports collection. Ports consist of makefiles and patches, so everything is compiled from source. The flags are already fine-tuned for you, but you can also tweak them yourself with 'make config'. So most software installation is as simple as make && make install. If you're familiar with Gentoo's Portage, the Arch Build System or the Slack way, you should feel at home.
Enjoy your journey.
As for other UNIX-based systems, I don't really know. MINIX has always been at the forefront as a research OS with its microkernel approach, but I don't know much about it, or if it's really suitable for desktop usage.
