I’ve replaced Drive, Calendar and Contacts with my own installation of ownCloud. It’s great – I’ve got full two-way sync with my Android devices, and complete control over my own data.
I had never heard of ownCloud before, but it certainly looks very nice. The hardest part of trying to move away from iCloud/Drive/Dropbox is that there is simply no developer support for other options.
So while it is easy to move away, there are very few tools/apps that support other services. I fear we will find ourselves in a Google Reader type situation with Dropbox in a few years — all of a sudden we have a real reason to move, and every developer is scrambling to update their apps to support 1,000 new services that pop up.
It’d be much easier to just not have apps that solely depend on Dropbox. Much in the way that Ulysses III can use local storage, Dropbox, iCloud, and what have you.