Please do not use OwnCloud. We just got through several months of hell trying to pin down an awful issue with OwnCloud: It eats files. Somehow the server loses track of files and then syncs this state to all the other clients. We went through months of trying to track down what was happening to our business documents, only to be totally shocked in the end that it was OwnCloud itself. This is also a rather open secret among the OwnCloud developers and community. It's just something that happens sometimes, and the "solution" is to use another form of backup (thereby effectively negating the original purpose of using OwnCloud).
If you value the data you are storing, do not use OwnCloud!
I've heard this sort of thing a couple of times but never seen any evidence (specific examples or ways that reliably reproduce the problem, online discussions about it (i.e. on support forums), and so forth).
I'm considering a small OwnCloud install for myself[], freinds & family, so any info you can pass on would help evaluating that. Also, if you can recommend alternatives to look into, that would be nice. What do you now use instead?
[] Actually for my own backups I'll be sticking with my own set of rsync paths that have worked well for quite some time, but OwnCloud will be for sharing stuff and giving people an easy backup option to encourage them to actually have an off-device backup (it'll reduce the "I've deleted X can you get it back for me?" and "my laptop died with all my stuff on it, is there anything you can do?" requests I get!).
Unfortunately, it's impossible to reliably reproduce this problem. If you Google a bit though, you can easily find lots of results for files that magically disappear[0], and not just during upgrades. We were able to pinpoint OwnCloud after making a private folder for one of our guest laptops (which we normally use for interviews and visiting clients, that sort of thing), and then leaving the laptop disconnected for a few weeks. This was a folder no one had access to, and files disappeared and then later this state was synced to the client. It is definitely a problem with OwnCloud.
If you value the data you are storing, do not use OwnCloud!