Not the greatest game ever, but still provides a few hours of fun.
The game takes place two years before the movie, but with roughly the same storyline. There are a few missions as an RDA soldier and a few as an Avatar, but an hour in, you are given a choice between the two factions. If you pick RDA, you will be treated to an alright third person shooter. The quests are all fetch quests, and mostly involve you finding "harmonics" and helping soldiers fight the Na'vi. The weapons are mostly generic types, but the developers were very good at making each one have it's strengths and weaknesses. They are upgraded on an XP-based system.
The RDA also have many vehicles to drive. The cars (Buggy, ATV, gun truck) are a bit twitchy but are fun to use once you get the hang of it. The helicopters are very fun to use. The controls could be better, but they are designed for ease of use and that is achieved. A small detail that I enjoyed with the chopper is that when you fire a missile, it falls out of it's holder and then ignites its engine and races to your target. My only complaint with the chopper is that with the basic controls, it is impossible to strafe the ground.
The Na'vi campaign involves fetch quests as well, but is more of an action game. You are given melee weapons, as well as a bow and a machine gun. The animals you use are not as exciting as some of the RDA offerings, but are still fun nonetheless.
The game is very good graphically, especially at night when the jungles of Pandora light up in bright colors. Explosions and fire are very satisfying, and the flamethrower effects are the best I have ever seen in a game.
Another small detail worth mentioning is that on your first playthrough with a character, the game saves a separate file when you reach your decision about which faction to join, saving you an hour on your second playthrough.
Trophies (or achievements) are not deep like some others, but do provide a fairly attainable Platinum, if you put a few extra hours in.
Each map you travel to offers a number of "Sector Challenges," which range from killing a certain number of enemies to activating ammo pods around the map. While the objectives are cut and dry, it is fun to "clean sweep" each sector and earn your trophy.
Multiplayer offers fairly standard modes and is pretty fun. The faction balance is done nicely.
Over all, don't let the bad rep of this game turn you away. The RDA campaign actually feels a lot like Lost Planet 2 (even some of the sound effects are similar). It is incredibly cheap and fans of sci-fi games or the movie should at least give it a rent.
UPDATE:
A part of the game that really pissed me off was the choice section. At the first choice, you are told to shoot an avatar and the avatar urges you to shoot the RDA soldiers. There is a Na'vi symbol over the avatar's head and an RDA symbol over the soldier's head. This is odd, since it isn't telling you which side you will take but rather clarifying the already obvious distinction between giant blue people and regular people. Later on in the game, you are given the option to switch sides again. There are two circles carrying the symbol of each side. The Na'vi circle displays text saying "choose to stand with the RDA in it, and the RDA symbol says "fight with the Na'vi." The damn symbols are mixed up! I wasn't sure what to follow, the symbol or the text, so i followed the text and ended up switching sides to the Na'vi, missing outon the great vehicle action sequence I was working to. On top of this, the checkpoint doesn't save. I thought I should warn you guys about that.