I'm planning on setting up one of these in a month, and I'm considering FreeNAS and NAS4Free. I'm very interested in comments from anyone with experience with both.
I've used both, migrated between them, and support instances of both for different clients.
tl;dr: NAS4Free better adheres to the UNIX philosophy of "do one thing and do it well". FreeNAS does not - it does more stuff. Depending on your use case, either one of them can be a help or a hindrance.
Both of them essentially solve the same problem, essentially the same way: Get a bunch of hard disks recognized by a computer, and use the ZFS file system and various networking protocols together in order to facilitate data storage. Both of them have the same advantages of ZFS (Data security, "datasets", good performance in software RAID, snapshotting, compression, volume portability) and cons (you'll need plenty of RAM [ECC RAM is strongly recommended], hardware RAID controllers are only useful in JBOD mode, adding disks later on gets weird, etc.). If the ZFS tradeoff is worthwhile for you, then you're in the right place.
Pros, NAS4Free:
--Runs better on lower spec'd hardware.
--Faster startup time and generally snappier web interface.
--Has all the core stuff (SMB, FTP, SSH, NFS, iSCSI), and notably, Transmission.
--"More Open" than FreeNAS with regards to licensing.
Cons, NAS4Free:
--Limited functionality beyond NAS stuff, i.e. no plugins, though there are a handful of tutorials for unofficial methods (I've personally set one up to run BT Sync and Plex, but it took about an hour and LOTS of command line fun).
--Update schedule is erratic.
--I've personally had some annoyances with their Samba implementation; it doesn't always respect "remember password" in mixed environments with mapped drives.
Pros, FreeNAS:
--Extensible functionality with plugins; there are multiple avenues for media streaming and automatic downloading (Transmission, SabNZBd, XDM, etc.). There's also an OwnCloud plugin which is very nice, and an Amazon S3 plugin that allows for real-time replication to The Cloud (tm) if that's worthwhile. Depending on the environment, integration with Active Directory is possible.
--ZFS Replication - you can have your datasets replicate to a secondary NAS somewhere else.
--In-UI updating, automatic or scheduled. This is a new feature in 9.3 admittedly, but it no longer requires updates to be manually uploaded or the NAS to be taken offline for an update to be performed.
Cons, FreeNAS:
--All those extra features come at a cost - you'll need to account for that when buying RAM.
--Plugin updates aren't always immediate when the source program updates; when some programs update internally, it's not always reflected in the FreeNAS UI.
--UI is more daunting at first go. Also, some things are a bit more quirky than they should be.
--iSCSI is a bit more complicated to set up than on N4F.
I personally like the FreeNAS route myself, but that's also based on my extensive use of plugins, because I'm trying to do "one box to rule them all" - FreeNAS fits that bill better. If you either don't care about your NAS doing anything besides speaking FTP and SMB, or you've got an ESXi server running around that does all your other server-like stuff and you just need an iSCSI target, or you're building a FrankenNAS and need to squeeze the most out of your RAM, then N4F is probably more practical for your use case.