Yes, maybe I should've made this emphasis stronger in my comment. Also I didn't meant to say that using owncloud is worse than using dropbox in security terms - but you should use your own (client-side) crypto either way if you care about data security.

For myself I use my selfhosted owncloud (running on a cheap VPS) with EncFS, encourage other users on that server to use client-side encryption as well and never lied about how secure their data is when they decide not to. But this isn't so easy as it should be.