I have a Iphone and will not replace it with an Android phone. I admit it the Samsung S3 is very impressive, along with a host of other android phones. If I was starting fresh I would definitely consider a Andriod based phone, but there are 3 reasons why I am not going to switch which would apply for anyone else that already has an Iphone
1. The Samsung S3 or Nexus 4 are not a night and day difference from the Iphone. All have an Easy to use Interface, a solid camera, tons of apps, and great coverage.
2. I have an Ipad, AppleTV, and a Macbook air that work Seamlessly together with the Iphone. I do not know of any Andriod based system that Unifies all devices like Apple does. Even if I got the S3, Galaxy Tab, HTPC, and a new laptop. I would have to spend many hours getting them to work even a fraction as well. With Apple I just have to sign in to iCloud.
3. Apps, Videos, and Music. Over the last 4 years I have had a Iphone I have purchased several Hundred Dollars worth of these items. If I switch, I have to start over again.