I've used Spideroak for a year or so, and believe for a single device its quite secure. But there is a gap that I haven't figured out a work-around yet.

Let's say you have 2 devices to backup, a laptop that doesn't contain too much secret info, and a home desktop that contains much more secret info. If both devices are on the same account, theft/loss of your laptop could very easily mean the compromise of your secret data from desktop as well, because both devices can access all files.

The web site has a FAQ to suggest to de-register the lost device and change password. But there is a natural time gap before you can accomplish this. A gap that could result in data compromise.

Double encryption of the secret data is an option, probably the best, but you'll need to work up some scripts to accomplish.

Just be nice if there was a way to isolate devices, with perhaps a master key access.

Or two accounts.

I find it very very weird if they want you to use the same private key on both computers, thats just bad design in my opinion, but the password for the private key should expire after a period of time i would believe and if not that too is really poor design, you should have to re-type the password for the private key after some period of time has passed...