In which situations is it advisable to opt for BSD systems instead of Linux?
For an everyday-user with new hardware Linux seems for me the natural choice if somebody is looking for an alternative to Windows. But when does it make sense to give the BSD variants a try?
I've always found the BSD's to be more intuitive. There are some different philosophies in BSD than in Linux. For example, Linux prefers GNU commands, while BSD opts for either classic BSD commands (which are similar, but often times have different options) or newly written ones, falling back to GNU when nothing else is available. Also, I find the BSD Man pages to be more comprehensive and contain more examples than GNU man pages, since GNU tends to prefer info pages (which I despise) for examples.
Many ISP sysadmins swear by BSD. They claim it holds up better under load, hasn't made as many compromsies for the desktop, and that it's networking stack is more efficient and less buggy. I don't know if those are, or are still true, but this is what i've been told.
Also, OpenBSD has a reputation of focusing heavily on security, and they have historically had a very good track record when it comes to security. They take proactive measures (developing new C Runtime library routines, for instance) to prevent security flaws before they can be written.
NetBSD has a reputation of running on just about anything. They have a long list of platforms they actively support. Linux, to some extent tries to do this as well, but typically only a small subset of these are mainline supported.
Finally, it often just comes down to personal preference. Do the guys you have or are going to hire know BSD? Do you personally like it?
There are also some reasons NOT to run BSD. If you're primarily a desktop user, BSD may not be the best choice. Sure, you can install most of the same stuff on BSD as Linux, but you won't find a "distro" similar to, say, Ubuntu which focuses strictly on the desktop. Also, some device drivers aren't available on BSD because they were written with GPL only licenses.
I'm told that the BSDs are more... coherent than the Linuxes. I've had long conversations with my sysadmin friend on why/why not BSD/Linux. Here's a link:
Having said that, I started using Debian in 2007, and I've never looked back! :)
One of the big areas that BSD has over Linux is licensing. Linux's GPL can make it difficult to use some differently-licensed features of other Operating Systems. The first one that springs to mind is ZFS.
Plus, BSD is a bit more mature operating system (being directly descendent from AT&T System V UNIX).
The commonly cited wisdom is that BSD is more useful for a server OS and Linux is more useful for a desktop OS. But don't take that as the gospel truth as lots of people have successfully used Linux as a server OS and lots of people have used BSD as a desktop OS.