COPS' main advantages are :
* No need for many dependencies.
* No need for a lot of CPU or RAM.
* Not much code.
* Search is available.
* With Dropbox / Owncloud it's very easy to have an up to date OPDS content server.
* 100% OPDS valid code (checked with http://opds-validator.appspot.com/ ).
* It was my first PHP experiment and so fun to code.
It's been reported as working on most web servers (Nginx, Apache, Cherokee, Lighttpd, IIS) and is used a lot on NAS (Synology, QNap, ReadyNas). More Info »
I personally protect my COPS catalog with Basic HTTP auth and HTTPS. It's secure enough for my needs.
On the OPDS client side I mainly tested with FBReader Mantano Reader and Aldiko on Android (these 3 also ask for user/password if you have protected your COPS catalog). I also used Ibis Reader with success but there is no support for password. Other users have reported COPS working with Stanza, Megareader, Shubook and Bluefire.
As I already said, I had never coded in PHP before so I'm not proud at all about the code quality. I'll probably start it again in the future ;).
* HTML5 / CSS3 interface with responsive design.
* Multiple Calibre database support in a single COPS install.
* Epub metadata update like Calibre Content Server (enable it with $config['cops_update_epub-metadata']) : If you fixed the author name / a tag / the serie name of a book in Calibre, then the epub you'll download with COPS will contain the fix.
* Calibre custom columns.
* Facets in the OPDS feed to filter book list (the only OPDS clients supporting it are Mantano Reader and Bluefire for now).
* Multilanguage : Catalan, Czech, German, English, Spanish, Basque, French, Haitian (Creole), Hungarian, Italian, Norwegian Bokmål, Dutch, Polish, Portuguese, Russian, Swedish, Ukrainian, Chinese.
* Certainly many other.