I dual-boot between Linux and FreeBSD, but I haven't used FreeBSD much. First impressions were that FreeBSD is trickier, because recompiling the kernel is really a must - whereas it isn't with modern Linux distros - but it's also much easier to get a kernel compilation failure in FreeBSD than it is in Linux. (In fact, I don't think I've *ever* had a kernel comp failure in Linux.)
Other than that, it's really just a matter of getting used to the BSD-style init mechanism, and the different device names. For example, under FreeBSD my modem is /dev/cuaa4, which the kernel sets up automatically. Eh? KDE's internet dialler doesn't even let me choose that as a modem device...