It made it SUCH a better movie. I finally got it and watched it on my PS3 and there are such more improvements.
They showed injuries/more blood, so it doesn't look like they are invincible figures and made it more realistic.
They also described the Silver Hair Gang's plot in the story more. (Silver Hair Gang is the name of Kadaj's gang.)
But the best part of the complete version is the extended Motorcycle Chase and extended battle with Sephiroth. My favorite part is when they show Cloud's reminiscence of him and Zack and how Cloud is now Zack's living legacy.
Some things I didn't understand though is Kadaj and them.
Aren't Kadal, Loz & Yazuu all supposedly larvae of Sephiroth's body diffused with power from the Lifestream? And with the Jenova cells they reunite with Sephiroth?
Also, when the dialogue is:
Aeris: "He's too big for us to adopt him."
Zack: "Sorry, friend. You don't belong here."
By "you don't belong here" did he mean he can't join them in the Lifestream/Heaven like you don't belong with the dead?