So for the slightly older iPhones, including iPhone4 and iPhone 4s, the parallax affect can take up some battery life. Not much, but with battery power so limited anything helps. If battery isn't a concern for you, maybe you just don't like the effect (which is my case) I feel like they didn't smooth it out enough to be cool. It seemed really jittery, so I was happy to find a setting to turn it off.
To turn it off go into settings:
Click 'Reduce Motion'
And finally turn it off! (It needs to be in the 'enabled' state to turn off)
Now on your iOS7 iPhone 5, iPhone 5s, iPhone 5c, iPhone 4s or iPhone 4 can turn off this blasted parallax effect that apple has deemed necessary to have out of the box on your newly updated iOS7 iPhone.