Systems:
This version, set two years before the film, tells the story of an RDA signals specialist named Able Ryder (whose gender and appearance is the player's choice). They have an Avatar body which assists them in navigating the landscape of Pandora. It is soon discovered that there is a mole in the ranks of the Avatars working for the Na'vi, and you are the one tasked with finding them... The game plays like a traditional third-person shooter, albeit with a bit more action/adventure elements.
This game isn't AWFUL unless you're playing for the story. The gameplay and graphics are awesome, but the story is lackluster and quite awful. Like...the ending cutscene is only around 1 or 2 minutes long, abrupt, and seems almost disconnected from what you were doing just 5 seconds before (In fact, there's a path in the game that you can take that *SEMI-SPOILERS AHEAD* gets a character killed, but in the ending he'll still be alive and even speak to you *END SEMI-SPOILERS*). Also, there aren't multiple endings even though you can make choices that change how the rest of the game plays out (see spoiler in in previous sentence), which is totally sucky. Also some plot elements are bastardized versions of things that happened in the film, and feels kind of tacked-on and poorly done.
Let me note that if you're looking for amazing night visuals, like in the film, YOU WILL GET AMAZING NIGHT VISUALS. The following screenshot shows that off, if you're curious to see if it holds up to the film. It totally does, and even I had to just stop and gawk even though I had seen the film several times before I played the game and none of the things seen were entirely new to me. The graphics are just crazy good, even though the game is kind of bad.
(http://img4.wikia.nocookie.net/__cb20100420084253/jamescameronsavatar/images/thumb/2/24/Screenshot0000.png/600px-Screenshot0000.png)
(http://img4.wikia.nocookie.net/__cb20100321162829/jamescameronsavatar/images/thumb/2/2d/GameScreenshot2.jpg/600px-GameScreenshot2.jpg)