Like many others, I use Dropbox to synchronize files among my computers and iPad. Dropbox is especially useful when traveling, as I often do.
you grant us (and those we work with to provide the Services) worldwide, non-exclusive, royalty-free, sublicenseable rights to use, copy, distribute, prepare derivative works (such as translations or format conversions) of, perform, or publicly display that stuff to the extent reasonably necessary for the Service.
This statement says the company can use your data pretty much as it chooses, a position that the next sentence attempts to mitigate:
This license is solely to enable us to technically administer, display, and operate the Services.
That last sentence qualifies the core issue by saying they can only use your data to operate their service. However, this language is ambiguous and therefore subject to interpretation. For example, perhaps Dropbox will want to scan your files to provide context-sensitive ads like Google. That would certainly fit within the definition of "technically administering" the service, as would many other activities that you may or may not find acceptable.
Advice to enterprise buyers. Dropbox offers a great service and useful free accounts, which is an attractive combination. Unfortunately, the terms of service do not offer adequate protections against sensitive data. For this reason, I suggest you discontinue use of the product for applications where privacy and confidentiality are mission critical.
In practice, however, Dropbox is unlikely to read your "stuff" or prepare derivative works, despite what's in the terms of service. Therefore, continue using Dropbox for everyday file transfers where you value convenience over an absolute guarantee of privacy.
Photo by Michael Krigsman