The S3 is a great phone (weaker iOS, but still great phone). I may go android (though for the convenience of the ecosystem with my mac computers and iPad am say 60% likely to stay and get iPhone 5).
Lots of other people I know are thinking about it. A few are thinking about it given customizability options with a more open OS. Many, like me, are simply frustrated at the small screen size the iPhone 5 is likely to get. To be honest, that is my only complaint.
So let's be honest
1. Apple is still the market leader in laptops, tables and phones.
2. The lead appears to be lessening in phones more so than other categories. The Nexus and S3 are (for some) finally now real alternatives (iPhone may still win, but they are plausible).
3. Competition is good for all as it will force Apple to keep innovating.
4. Apple may in future be forced to fragment on screen sizes for phones (perhaps not until next year) as they do for laptops and soon will do for iPad. The non-Apple market is responding to consumer demands for a 4.3 to 4.7 inch screen. Do you really think Apple knows best here? I'm not so sure.