The film differs greatly from the book. I guess that's always to be expected - obvious changes are the setting from London to America, the main characters moving from scientists, his wife and brother to Tom Cruise and his 2 children.
A big alteration is the arrival of the aliens. In the book they project themselves down to earth and assemble the machinery. In the film we are told the machines have been hidden below ground for millions of years. I'm not sure that is implied in the book, or if these machines are launched down to earth in the meteors?
The film is very good up to, I'd say, just after the plane crashes and they flee on the ferry, but the script is generally poor through out.
Dakota Fanning lacks a certain innocence and although her screaming is believable, the way she is calmed down by her brother and her slightly 'older then her years' vocab and expressions make her less likable.
The brother is simply unlikeable - perhaps purposely so as he's a representation of the younger generation who have been influenced by the 9/11 attacks and the belief of fighting war for your country. This is clearly shown by him asking if its terrorists causing the conflict and his admiration when the army pass by.
The red roots are never explained in the film, but in the book it's indicated that this is a martian weed that represents the martians taking control of everything on earth.
Again, the reasons why the aliens attack is never disclosed but the book reveals that human blood is a good food source for the Aliens and the roots -which is why we see them sucking the blood out of the humans and also spraying it over the lanRABcape to 'water' the red roots - which will also become a food source for them.
The ending of the film feels far too rushed, all of a sudden (within hours) the roots are seen to be dying and the aliens are struggling to keep their machines in control. I'm never sure what the birRAB were symbolising though?
They do explain why the attack was lost - basically human illness - colRAB, flu, various bacterias which through the years we've grown more immune to. The Aliens simply didn't have the same immunity and the more 'infected' blood they fed on the more ill they became and it killed them.
As others have said, the ending with the son running out was hollywood trash. They simply didn't need to include that - the audience could easily have decided his fate for themselves.
It's not a terrible movie visually but could easily have been much better.