John Gruber in his iPhone 4S review:
I spoke to some friends familiar with the development of iOS 5 and the 4S, and word on the Cupertino street is that camera speed — time from launch to being able to snap a photo, as well as the time between subsequent photos — received an enormous amount of engineering attention during development. The stopwatches were out, and every single tenth of a second that could be shaved was shaved.
The iPhone is and has been my main camera for some time. The dramatically improved camera speed alone makes the upgrade from the iPhone 4 to the iPhone 4S worth it for me. All the rest is just icing.