SpiderOak looks great. The only issue is that, since the client is not Open Source (or is it?), you have to trust what it does and you cannot verify it in the code.

To my knowledge it is not. For this reason, I stopped using it, despite their otherwise very nice approach generally. I have been doing without cloud storage for my most important files (financial documents, etc.), so I'm pretty interested in Filerock, having found this thread.

I naively attempted to roll my own mash-up for this purpose (https://github.com/clebio/duplicitySync) and was not successful. The combination of local encryption, incremental backup, and _synchronization across multiple computers_ was the breaking point.