The alternatives try to address the biggest problem of Dropbox: the Dropbox company knows the content of my files. That in turn means that various US agencies can browse and analyze my files without my knowledge, approval and a warrant.
Sure, you can just put a Truecrypt disk image in a Dropbox folder, but you're missing the granularity of Dropbox (e.g. to sync certain files to my SSD) and that granularity also affords you protection against race conditions (if both machines have the encrypted drive mounted and are writing, sounds like trouble).
Not an insolvable problem by any means, but not one that has yet been solved either.
Unfortunately the one time I installed it the user experience and interface was miserable..