Great solution. I have been using it on and off, started few years ago but it consumed 6-8% of resources on Mac but I gave it a try recently again and I must say it's way better, consumes 1-2 person when uploading and 0.0-0.5 when idle, which is amazing given that this is one of few relatively more secure sync or share solution out there. This does encryption of data on your machine, rather than sending unencrypted or through SSL and then encrypted. In case of DropBox all they say it's encrypted on their servers, but where is no guarantee how the data and keys are stored, so you got to have something like boxcryptor or cloudflogger. Keep in mind that the much celebrated SugarSync (now virtually unaffordable and outpriced for most of average people) is same way insecure as dropbox, no local AES encryption, just in transit and final encryption on their server. I don't know about Googledrive and Skydrive, but the only other service similar to SpiderOak is Syncplicity. Syncplicity is much easier to use and has nice new interface on iPad (but sorry, secure editing of documents on iPad doesn't work in case you are excited about that part), however, only SpiderOak allows you to fine-tune the sync through their interface. So while the SpiderOak interface requires user manual (I had to check a few things), it's perfectly configurable, almost overwhelming, but very useful for power-user, and - hey - you don't use the interface every day - you set it and forget it. The linking with devices and machines is the most tricky part of SpiderOak.
The other part I am a little bit puzzled is where they state on the website following:
Important Note: When accessing your data via the SpiderOak website or a mobile device, you must enter your password which will then exist in the SpiderOak server memory for the duration of your browsing session. For this amount of time your password is stored in encrypted memory and never written to an unencrypted disk. The moment your browsing session ends your password is destroyed and no further trace is left.
So I am little bit puzzled reading this, but of course after re-reading this again, this is nothing unusually compared to any other online service when you login. They just put this disclaimer as they claim on other place on their website how they have "zero knowledge" and absolute privacy.
In any case - if you are really concerned about privacy, I wouldn't store bare data in the cloud. In that case using something like Coudfrogger or Boxcryptor is better and if you want total security, you have to go with something like Truecrypt (but issue is obviously you have to update whole encrypted volume every time its content changes, which could be unrealistic in cases with volumes over 1GB and slow network connection under 1mb upload speed.
I think it's really overdue the cloud storage providers come with better ways to store the data and provide option if I want to store the encryption key with my data online or not (and therefore forgo accessing it online or not).
Overall I think SpiderOak is a good solution for sync if you are looking to sync folders and do backup rather than having 1 folder only as it's in case of say Dropbox.
The cost is reasonable, compared to Sugarsync new rates.