I am trying out SpiderOak at the moment with the free 2 GB account, and I think there are a lot of things to like about it.
I love the zero-knowledge policy. Being able to encrypt the data on your machine with your own key before it gets sent to the server was one of my reasons for choosing CrashPlan, so I am happy to see this in SpiderOak as well.
I am not sure it's entirely fair to compare SpiderOak to DropBox. DropBox focuses on easily sharing a single folder across multiple machines, whereas SpiderOak (and SugerSync for that matter) are somewhere in between DropBox and backup services like CrashPlan.
DropBox is so easy to use that anybody can figure it out, but you pay for that simplicity with less security and flexibility. SpiderOak can provide syncing as well, but having to first back up the folders on each machine and then setup a sync between them is just way more complicated.
It has decent support for working as a backup solution as well, but lacks stuff like being able to run when idle, scheduled backups, running as a service, backing up more than the current users data, etc. that you get with a dedicated backup solution like CrashPlan.
SpiderOak seems to be based on some really good ideas, and with a little more work it could end up being a great replacement for both types of applications -- if they added something like the "Magic Briefcase" from SugerSync to give people an easy initial sync folder, and decent idle/time scheduling like CrashPlan
On a side note, I think they should consider making a small tray application to show some basic info and allow you to launch the full GUI, because while the backup engine is only using around 30 MB of memory here at the moment, the GUI uses 70 MB minimized to the tray doing nothing.