Is it worth it if you have already played through XCOM Enemy Unknown?

Yes!
The basic game has been expanded to include many elements such as the EXALT agents and new weapons / skills that can be produced only with MELD.
You can choose whether you prefer his soldiers Genetically a little dot, or if you put him in a bold robot arms and me goes out brute firepower.
In addition, the individualization of the soldiers has been revised and expanded.
There have been added new maps and there are also new missions that provide a bit more variety.
If you like turn-based games, you should consider purchasing necessarily ... the selection of such games on the console, unfortunately, not so great.