>but for zero-knowledge guarantee I'm OK with that.

How can you guarantee that when the client isn't open source?

You don't.

Which is so disappointing because for a moment there they were getting my hopes up about Spideroak, but I can't get behind that in light of their proprietary client. Not even interested in trying it, its a non-starter in my books.

I can't. However, unless you built literally your entire system from source code and all of the programs that run on it and analysed that source code yourself personally, you can't guarentee that about anything.

It's a matter of degree of trust. This is what Spideroak's business is built on. Sure, I'd like it if their client would be open source, however in the mean time they're still a better option than Dropbox for security conscious people and they have been working for a while to make as much as possible open source. [0]

[0] https://github.com/SpiderOak

Spideroak is great and I was lucky enough to get in on their unlimited storage they offered a while back. I still only use around 200GB so far, but with the amount of photos my wife takes of the kids it is rapidly expanding.

Condoleezza rice joining the board had a lot to do with it i think. I know a lot of people who cancelled dropbox then. Conscionable people cannot do business with someone who lied to get the Iraq war started. Also, there's no doubt she has NSA connections being a former national security advisor.