... main advantage over FreeNAS is that since FreeNAS likes to use ZFS you can't easily expand storage ...
You can add a mirrored pair of drives quite rapidly.
You can have one or more storage pools. You can control what goes in to each pool (e.g. by different shares).
A storage pool is supported by one or more vdevs (virtual devices), which can be of different types. Data is spread uncontrollably over the vdevs.
A vdev contains disks (or disk partitions), and can be of two main types:
* mirror - every disk (from 1 upwards) contains the same data.
* raidz, raidz-2, raidz-3 - the data is spread across the entire vdev, using 1, 2 or 3 drives worth of space for redundancy. The size of the smallest drive currently in the vdev controls the maximum amount of space available. You cannot add another disk to a vdev, only replace the disks one by one with a bigger disk and resilver it. Until you replace the last disk you don't gain any more space.
So, to expand a pool the easiest and smallest resilient way is to add a mirror vdev containing 2 disks.
I have two FreeNAS servers - a HP Microserver with 4 * 2 TB disks in a RAIDZ vdev (~ 6 TB available, used for backup and experimentation, currently broken) and a FreeNAS Mini with 4 * 6 TB disks in a RAIDZ-2 vdev (~ 10 TB available, my main NAS). With hindsight I should have set up the FreeNAS Mini with 2 mirror vdevs, each containing 2 disks.
I think FreeNAS/ZFS are wonderful. ZFS's checking parity information whenever data is accessed (and scrubs to force access) helps to detect rotting disks before you loose data. Compression can help (depends how compressible your data is - I get 1.40 on a NFS share used mainly for email, but 1.00 on a big MP3 collection). Snapshots (allowing you to recover deleted or corrupted files) can be a life saver. Replication allows a copy of your data to be kept on another machine, possibly off-site. I also back up the important stuff to a rotating pair of UFS formatted external USB3 disks with rsync.
FreeNAS 10, still in beta and not going to be released until it is ready, is looking pretty good.
There are downsides
* ZFS eats memory. 8 GB is the practical minimum.
* If you use NFS or iSCSI you will probably want to use a (possibly mirrored) SSD as an intent log (ZIL).
* People assume that a L2ARC cache SSD will help performance. It usually won't, and will instead impair performance because memory which could be used to cache data is instead used to hold information about the L2ARC content. Measure your workload before you buy. I did not, and wasted my money.
* Deduplication really gobbles memory and how much bit-identical data do you have anyway? Really only appropriate where people are storing many VMs of the same type on the NAS.
When I can afford it, I intend to build/buy a machine with many more SAS/SATA ports and drive bays and a lot of memory. Plus of course a lot of disks.