Just curious about what the advantage(s) are of using FreeNAS or OpenFiler over Ubnutu for a NAS box.
Is it reliability, performance, both?
I'd say the main advantage is ease of configuration - using something that's specifically designed for the role of a NAS server, as opposed to an all-around Linux distro. FreeNAS and OpenFiler also have features within easy reach that stock Ubuntu doesn't.
I use Openfiler at home, and straight NFS/CIFS at work.
Openfiler is all about the easy. It's easy to install: drop in the CD, boot, done. It's easy to configure: create and manage LVs, VGs, share, permissions, even update the OS, all from a web interface.
Doing it straight is all about the flexibility. I can (and often need to!) fine tune all sorts of stuff when I do NFS/CIFS directly. That often takes arcane knowledge of the Linux command line, NFS/CIFS config files, even firewalls. But it's my job to know how to do that.
It's easer to install and use, You put the CD, Enter, choose the disk, enter your password, install, reboot and voila you just have to go to the web interface to finish the configuration, no need to edit the files or install something else.
The reliability difference will be mostly negligible (openfiler is based on rpath linux and freenas on some variety of BSD), the performance difference between those and ubuntu will depend on entirely what you mean by ubuntu, if you use a minimal ubuntu install with just an md array, lvm, a samba install and the iSCSI Enterprise Target daemon, performance will be similar (the two dedicated distro's may be slightly faster due to kernel customisations etc.).
The other thing is ease of setup, on openfiler, its 20 minutes in a web gui, in ubuntu you have to manually configure, mdadm, lvm, samba and ietd. (It's a good hour at least).