I read the book version before seeing the film and was quite impressed but the film changed a lot and not to the films credit. Firstly, the biggest change was the Independence Day speech part before the final attack above Area 51. In the book the Americans have gathered together anything that can fly, from P-51 Mustangs, Phantoms, Starfighters, Sabres, Harriers, F 111s, F 14s, Migs, etc and these are to be used on a suicide run to distract the Aliens from the main attack of the F 18s. Most of these pilots are very depressed and fatalistic so the US President gives them his great Independence day speech to give them courage and hope and it works. But in the film it just comes across as a bit of over the top jingoistic spin and then swarms of F 18 go in to the attack.
Secondly it lacked something, perhaps a ground element, I think the film would have been ten times better with Alien AFVs (armoured fighting vehicles) attacking the base on the ground, pushing back the human defenders: USAF men, Police, Soldiers, civilians, Marines, National Guard, Secret Service, etc being pushed back in to the base but fighting fiercely. (See 55 Days in Peking to see what I mean). These AFVs could have been stopped ultimately by the computer virus too.
Also, what is with all the Ground controllers shouting frantically and loudly in to their radios at the pilots?? I am sure the last thing a pilot travelling at 500mph+ in a dog fight needs is a ground controller shouting down his ear! The director should have re-watched Star Wars, remember when the X wings and Y Wings are attacking the Death Star and the ground controllers are quietly waiting and watching? You could feel the tension.
This director writer team has a gift of getting a great idea and making it ok, remember Stargate?
Great special effects, great cast, good story but poor direction and realisation. Watch and enjoy but dont expect a lot!