From a user perspective, probably not much. The command options are a bit different, due to the different lineages of BSD and GNU/Linux. Linux supports more hardware (with a few exceptions).
Underneath, Linux is generally quicker to try new things and new approaches. The BSDs take more time to incorporate new features, but many feel that they do a better job of engineering these features, with the side effect that once something goes in, it's "done right", with less of the, shall we say, "experimental" nature Linux sometimes has.
Documentation is very important in the BSD projects, and I've found the man pages to be great sources of information. There are usually other sources, too, like a "handbook"-style general guide, the sort of thing that gets relegated to a wiki with some Linux distros.
Probably the biggest concern for BSD on the desktop for the casual user is the increasing tendency for developers to target Linux specifically and not worry about portability. The BSD teams have had to write compatibility shims to emulate certain Linux programming interfaces to get things like window managers to run. Hopefully if BSD gets more popular (thanks systemd
) this will become less of an issue.
I haven't used PC-BSD yet, but I plan to try OpenBSD (which I have used for a long time) and FreeBSD or PC-BSD on my Toughbooks. I use Linux too, and while I prefer the BSD CLI environment, in the end the best tool for the job wins.