Inspired by a blog post on Groovy Skills International I also upgraded my FreeNAS installation with my ownCloud 4.5. Advantage: with this, files can be synchronized automatically and are then available on different computers as well as over the internet with different smartphone apps or a web interface. The latter requires that the FreeNAS installation can also be accessed externally, which is only recommended if the system is really securely configured. However, the advantage is that by using the plugin jail, the actual IP of the NAS installation is not made externally available. If you want it for sure, you can use a separate jail for ownCloud.
At this point I do not want to go any further into the installation of ownCloud 4.5 at FreeNAS in the Plugin Jail - the aforementioned blog takes care of this very well. What got in my way was a missing configuration of the DNS server as well as a missing editor (besides vi, which I don't like :)). Both can be solved easily, nano can be easily upgraded via the FreeBSD ports.
The advantage of having your own solution: you are not dependent on the services of external services such as Dropbox, where data is always moved to the cloud and it is unclear whether it is really "secure" there - your own hard disks are then more trustworthy. Disadvantage: existing data cannot be transferred to the ownCloud - the system requires that the files can only be read and written by the web server user. This also means that the data in the ownCloud cannot be accessed without further ado, unless you use the corresponding user.
If you want to set up a FreeNAS system with your owncloud, you can do this on the basis of the HP Microserver N40L Shopping List for FreeNAS do.