In my mind Dropbox became a company not worth supporting when Rice joined Dropbox's board (http://www.drop-dropbox.com/). Personally, with a board member who advocates warrentless surveillance it seems unlikely that we share similar views on the security of my data, and I wont be using their service.
Feature-wise sync.com looks interesting, but there seem to be very few users out there. I would be worried that they disappear when they run out of VC money.
I switched all my data (about 250Gb) to them from Dropbox a couple of months ago and it has worked out well enough.
Features:
- servers not based in the US
- encryption
- privacy
- open source (thanks lima)
- multiple libraries (like multiple separate Dropbox folders)
- nice file manager for unsynced (parts of) libraries
- price
- payment options (Bitcoin)
- supports more OSes
- one can run one's own server
Cons vs Dropbox:
- syncing problems not always obvious
- some UX issues
- photo support
- selective sync cumbersome (if not using libraries)
- no LAN sync
So I definitely wouldn't store very sensitive stuff on SpiderOak either but I guess it's slightly better than dropbox.
Totally distributed, works like magic. Being distributed means you do have to blindly trust a third party, but also that don't have to worry about $ per megabite. For example, one of the machines I have in my Syncthing network is a Raspberry Pi with a 3TB drive getting a backup of my laptop $HOME and important stuff from other machines all the time.