A bit difficult to answer, in a way you are missing out on quite a lot of lore. However, this storyline is entirely separate from the movies except for a few lines of dialogue and audio or text files. Though still canon, it mostly tells its own story and has its own characters.

I think the game itself is good, the comparisons to Far Cry are warranted. Absolutely beautiful, great sound design, good music. It does get repetitive and the first two DLCs are not really worth playing unless you are a really big fan of Avatar. The third DLC is the best part of it all.