From what I've seen, Owncloud doesn't provide encryption (files are lying in the clear on the server); furthermore I didn't dig much into the web app part, but I wouldn't bet it to be extremely robust security-wise.
well said, they don't provide any encryption - and I have not found any public cloud services that could solve the browser encryption issue. If you are willing to experiment with public services, my first bet would be tresorit (however, they have limitations in compatibility until august, when upgrades are coming). I also tried wuala and boxcryptor, but they just did not work for me.
http://doc.owncloud.org/server/5.0/admin_manual/configuratio...