Most of us have a serious need for file storage and sharing. Sometimes, we also need assured security. I love DropBox for my files, photos and videos. However, I am not ready to store sensitive data at DropBox. There is a great article regarding DropBox and security that may help you identify important questions that you need to ask prior to plopping your sensitive files into DropBox.
http://www.techrepublic.com/blog/security/dropbox-convenient-absolutely-but-is-it-secure/5618
I think Cloud technology will become increasingly important in storage and access of data in the future, so I share your concern with security and stability.
http://www.latimes.com/business/technology/la-fi-tn-cloud-privacy-rights-20120424,0,3469998.story