I've been running Freenas for a couple of years now. First i ran 3 disks in a RAID 5 with a few extras for temporary storage.
I really like the reliability of it, it's rock solid, and once it's set up, it's fantastic.
The basic setup is very easy, especially if you install to a hard drive or CF card (i chose the latter). However, i tried for a while to get it to boot from a USB thumbdrive, and gave up. There are now new instructions on how to get this to work, but my CF setup works fine, so i haven't tried again.
Adding drives, creating RAID arrays, etc is very simple. Setting up Samba/CIFS is also very easy. This means you can quickly set up a server that plays very well with a mixed Mac and PC network.
When you start to customise a bit more, you can run into trouble however. I recently tried to upgrade my Samba transfer speed by tweaking settings, and ended up having to reset to factory defaults and restoring the config from a backup (this is actually easy to do, but i don't think that it should have been a necessary step).
Freenas is a bit fussy about hardware, it doesn't seem to like the onboard Nvidia NIC that comes with my ASUS motherboard, as i said before, it doesn't make using a USB thumbdrive easy.
Recently i've upgraded the disks and changed to 1TB drives backing up from disk to disk using the included Rsync, as the whole RAID 5 thing scared me (i worried that if it went wrong, the risk of making a mistake and losing all my data was too great). BTW, Rsync is ridiculously easy, in contrast to my experiences with trying to use it with Windows.
Support is through the forum on Sourceforge, and if you don't ask questions in the right way, you will be ignored. People are very helpful if you make the effort.
Finally, is it worth the hassle? Absolutely, i have a reliable server that once set up runs and runs. I use it for backing up various machines, as well as holding my photo, music, and video libraries, which stream to various machines. I touch the config about once a year, to upgrade to a newer version, or add disks, etc, and between those times i never need to worry about it.