First off the game was not based on the movie, it is based around the movie. The same world, the same species etc. Also some of the things put in the game made it into the film like some vehicles. If your looking for a mirror of the film then your not going to get it IMO.
I found it an absorbing and wonderful world to play in whether you choose to be the na'vi or humans. Each race has a different storyline by the way, so the game wont be the same for either side after you have made a choice. Also if you are human then the world is a hostile place indeed.
I have played the game through as the na'vi and i finished it in around 12 hours, im expecting to finish it in around the same time as a human. This has given me 24 hours of playing the game straight through without doing the side quests of finding a certain amount of things like A-Pods or killing a number of enemy per level. Then you also have the side game of Conquer which is a light hearted take over the world game but does have a bearing on the main single player game with added bonuses.
All in all i reckon for only 15 pound you have about 40 hours gameplay out of it and for that i cant complain.