The one thing that makes their privacy weak is: The software involved in the encryption/password handling is not open source. We have only their word for it that they are not snooping or letting anyone else snoop.
If you're willing to do the extra work, you can get a cloud service like Dream Objects, and use software like duply/duplicity to store your files online and encrypted. You may lose some flexibility, though.
I encourage you to look into borg backup[1][2] which appears to have replaced duplicity as the de facto standard for "robust backups that the provider knows nothing about".
This is really the direction you look for providers[3] to go in - giving you a blank slate to write whatever bits you want to and allowing you to control the encryption with your own tools.
If you point borg (or duplicity) at even the most privacy-antagonistic provider, they still have nothing but gibberish.
[1] https://borgbackup.readthedocs.io/en/stable/
Rsync.net have also been very good and competitively priced, as per the above link. I can recommend them as well. (no affiliation with either product).
EDIT: I only use the free version though, so maybe the paid plans have better speeds.
Now they also have a galaxy of related products (IM, collab, etc) which more or less integrate with ONE, although I haven't really used them.
No it was actually a couple of years ago, more than 3
I might give it a try.
FYI, the SpiderOakONE app and the backend storage service received a refresh in 2016, and another one is underway right now (now all needed libraries are Python3 compatible!)
The existing UI is oriented toward power users, and is a bit complex for most people. The upcoming refresh simplifies many things while retaining the flexibility under "advanced" settings.
I've been using SpiderOak for years without noticing any bloat or performance issues with the background service. On the contrary, I was often surprised how little space I'm using in spite of the fact that they store multiple versions of my files. It doesn't hog memory or bandwidth or CPU at all.
The UI is indeed a bit weird and its performance can be erratic sometimes, but it gets the job done and has a lot of useful features.
Most importantly, SpiderOak has reliably protected me from losing data and I don't have to babysit it. It just works.
(I'm a happy paying customer. No affiliation with them whatsoever)
Then, one day, my account got full, and I couldn't delete anything unless I got some more free space first (see the problem?). I believe support gave me a few extra GB just for the deletion, but that didn't work either and I decided to stop using the whole thing. That's when I switched to attic/borg, which is much superior for my use case (backups).
By who?
My only experiences with them have been horrible. Their client is buggy, slow and horrible and their support left tickets for literally months until they basically gave up and gave me a refund.