I was prepared to embrace SpiderOak as a more secure, better-suited-to-me backup/syncing service than Dropbox. I thought I'd like the ability to sync any directory/folder, and not just items under /dropbox.
I've been using Dropbox for a few years now, and I recently installed and ran SpiderOak on my Debian Squeeze desktop.
While the SpiderOak software seems to be undergoing fairly consistent improvement, I found it hard to configure and use, and when I unknowingly exceeded my 2 GB file limit, the service basically broke and I couldn't seem to either pay for more space or get access to bring the amount of data I had on the service under the 2 GB limit. And yes, I did contact SpiderOak for help.
Dropbox is extremely enthusiastic about supporting Linux, the /dropbox "limitation" makes it easy for me to regulate what I do and don't store with the service (though I'd like the option of selective syncing across the filesystem like SpiderOak).
In the end, it was a combination of service, reliability and software -- I really like the way it works -- that keeps me using Dropbox. I suppose you can throw in familiarity.
Had my SpiderOak experience gone better, I'd probably feel differently (or indifferently).
And now that Dropbox has doubled the amount of data you can store (or halved its prices, depending on how you look at it), the service is more attractive than ever.
It certainly makes my work across multiple computers a lot smoother and trouble-free.