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.
I have several friends living in China. They use Chinese version of Yelp, Facebook, etc. They are aware that the government spies on their online activities. But they use these services anyways, albeit with much care. I believe most Americans easily forget about this.
No, the web access feature is opt-in; you have to input your spideroak credentials so that spideroak client code running in their datacenter can decrypt your encrypted datastore and make it available via the web interface.
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.