Dropbox is primarily a sharing and synchronization tool: using it for backup is very expensive.
IMHO. YMMV.
Edit: They now match Spideroak's pricing. If anybody from Spideroak is reading this: you have an awesome service, but here are a couple of things you might want to do. I like your commitment to privacy and hence use it, but sadly, IMHO it's not easy enough for my spouse to use, and I wouldn't recommend it to them.
1. simplify the client.
2. sharing is not intuitive
3. default setting should be that all folders are synchronized: the opposite of the current one.