Few mainstream services are going to accept data they can't recover for a user when they forget their password and their computer failed. Which pretty much precludes client-side encryption.
There is a small perceptible delay when i try to use this(rarely).
That said, their UI is comprehensive feature-wise and not as user friendly as Dropbox UX.
Original promo announce: https://spideroak.com/blog/20110330182326-eric-brian-and-wor...