I truly believe that Dropbox is a lousy solution to a problem created by ignorance on the part of the OS vendors. There is absolutely no good reason that the computers of today cannot share media seamlessly between themselves, without requiring the use or involvement of an external third party - the only reason its necessary to have Dropbox now, is because OS vendors fell asleep at the wheel and forgot that they're producing Operating Systems.
I think this will change, though - OS vendors are going to catch up to this market, and Dropbox will become less and less important as the OS starts to do its job again, and lead on these kinds of services.