I use both OpenBSD and NetBSD alongside several Linux machines (which it would be fair to say I am more familiar with). Both are good quality operating systems suitable for a wide variety of tasks (atleast IMO) including file serving (be it smb, ftp, web, nfs etc), networking (including packet filtering, NAT and traffic shapping) and just about anything else you can do with a modern OS.
As allready mentioned the various different projects derived from BSD are just UNIX like operating systems (in the same way that Linux is), all be it with their own querks and subtleties (perhaps one of the most obvious being the license agreement). If you have a passing familiarity with using command line based Linux systems (Slackware comes to mind) then its really not a big deal adjusting to any of teh flavours of BSD, the documentation is also relatively plentiful and easy to find.
If you have any specific questions feel free to ask, otherwise I am just going to ramble on for pages of directionless waffle
Regards
eddy