The NSA doesn't have backdoors into Dropbox. Dropbox has had the highest rating from the EFF regarding user rights, transparency and privacy.
https://www.eff.org/who-has-your-back-government-data-reques...
https://www.eff.org/who-has-your-back-government-data-reques...
EFF has in no way verified the privacy policy is actually honored or that back-doors aren't installed in the companies' datacenters.
And that Dropbox was one of the companies in the whole PRISM scandal.
I still use it, mostly to transfer files between my devices, but there's no way I'm gonna be using it for anything sensitive. Everything sensitive goes to my organization's ownCloud installation which I completely trust since... well, I'm the one maintaining it.