Decent Film, But a Poor Adaptation of the Book
As much as people talk about how much Cruise's character (Cage) was changed from the book, others like Rita Vrataski and Ferrel were changed even more.
While Keiji Kiriya is a young Japanese man who volunteered for the UDF and is a skilled but untested soldier and William Cage is a cowardly middle aged white guy who is completely untrained in powered suits, this is one of the less severe changes from the source material.
The movie does not reveal the reasons for the mimics purpose on Earth (they are terraformers) or why they're called mimics (they mimicked starfish in the ocean to take their shape) in the first place.
Rita meets Keiji while he is dying of wounds in his first battle. She asks him if the green tea after a meal is free; this is a code phrase to see if he is looping. She then tells Keiji that he is dying, but she will protect him from the mimics until he does; only then will she take his suit battery. While Rita is slaughtering mimics, Keiji kills a server mimic which causes him to start looping. The server and antenna mimics send a signal back in time so that they can attack again 30 hours earlier to do better in battle. The loops will only stop if the server and antenna mimics are all killed in rapid succession.
In the film Rita shamelessly steals Cage's battery after he is wounded; Cage then kills a mimic which bleeds on him causing the loops. The loops in the film stop if the soldier gets a blood transfusion. What?
The military is one of the film's antagonists, while in the book the military is trying to find out the causes of the loops Rita is experiencing. Farrell is much different in the book, he is devoted to his troops, in the film he seems to care nothing for them at all, especially Cage.
The book has a tragic but hopeful ending after Kieji and Rita battle to the death, but the film has a sappy happy ending in which there is zero body count after Cage loops back two days prior to the battle.
The biggest change from the book is that Rita is revered by soldiers everywhere. Her ability to coordinate with air strikes then mop up mimics with her battle axe inspires everyone on the battlefield to give 110%. This is completely absent in the film.
This film is the Starship Troopers of the 2000's. Just not as campy.