No nothing major hold back mobi support (only my laziness). I'll work on it this weekend.
From my point of view it's not an alternative to Dropbox as I use it with dropbox.
I have a laptop with Calibre and my Calibre library is located in my dropbox folder.
I have a VPS with Dropbox and COPS installed.
So each time I make a modification on my laptop's calibre library it's automatically synchronized onto my VPS (thanks to Dropbox) and my OPDS catalog is always up to date without having to do any manual stuff (as it reads directly from the Calibre database).
I hope it's clearer.