I've used SpiderOak for quite a while, and I've found it to be a good service. If what the company says is true, it's very secure. The client seems reliable enough to actually perform the backing up, although its functionality could be improved. It automatically monitors and backs up your files while it's running, but it doesn't run on OS start-up by default. You can also access your files via the website, which is useful if you aren't at your computer.
The standard 2GB of free data is great for a moderate amount of documents, settings and the like, but it's not really well suited for music, videos, and other large data sources. You can increase this to 10GB by referring others to sign up. The referral program benefits both users; a referrer and referee both gain one gigabyte for a referral. If you want more than 10GB of storage, you'll have to pay for a subscription.
However, there is also a noteworthy problem with the synchronisation functionality: in the case of conflicts, you lose data without any notification, warning or confirmation. Because of this, I recommend you don't use the service for synchronisation unless you are willing to risk data loss.
I have noticed some bugs in the Windows client. Once I exceeded my quota. When I tried to remove some backed up files and configure the client to stop backing up some folders, it would not let me because I had exceeded my quota. Obviously this is very counter-intuitive, but I was able to work around it with a bit of fiddling around, including re-opening the client. Another problem is I've spotted the client running at 100% CPU usage for very long amounts of time on one computer.
Overall, I've been happy with the service since it's free and apparently secure despite the client software's shortcomings.
Similar to an earlier comment by another poster, here is a currently working method to create a new free account with 7GB of storage (instead of the normal 2GB):
- Sign up using this referral link. This causes both of us to gain an additional gigabyte. (Alternatively, use someone else's referral link. If you don't use any referral link, you and someone else miss out on the potential gigabyte each.)
- Use the Freedom4 promotional code:
- Sign in on the website.
- Go to the Account menu option.
- Click Buy More Space.
- Click Upgrade My Plan.
- Enter Freedom4 in the Promo Code box.
- Click Update.
- Ensure Monthly Billing is set to 6 Gigs: Free.
- Click Next.
- Click Confirm.