I am one of those using Dropbox to store my e-books, and I am perfectly fine with it. I don't use Content Server or Calibre2opds - I simply store my Calibre library in my Dropbox so that I have the same e-books on my both computers. This way, I also have a copy of books in the cloud - admittedly, it wouldn't be very easy to use because it mirrors the Calibre folder structure which is not a convenient way to browse through book, but at least my books are always available. Furthermore, I can use Dropbox search engine to find books if I know their titles or authors - again, far from the comfort that Calibre gives me, but just enough for use "on the way".
//EDIT
I've started reading more about Calibre2opds and it seems like a really good solution. Read the manual on publishing your library on Dropbox (scroll to the bottom).
http://calibre2opds.com/read-the-doc...lish-catalogs/