It isn't like we have many other options to go to if we want an electronic version of NEC.
My 2014 E book could use some improvements but overall I find it better for navigating through it then a typical pdf document.
references to other articles are live links that will take you to that other article, defined words have a symbol next to them that if you click on will show you the definition, sections that were changed from 2011 have a symbol next to them that you can click on and see commentary about the change.
My 2011 PDF has a terrible menu tree in comparison to the 2014 E book. I can get to a specific section much faster via the menu tree in the E book.
PDF's from NFPA often can only be loaded on to a limited number of devices without purchasing more licenses.
My 2014 E book is available on any device with internet access, all I need is my user name and password, and the app that it runs on, which is free.