The Main Reason for one wanting to Upgrade to iPhone 3GS is coz it's 3G Speed for an extra 100$.That's the reason why Apple called this the iPhone 3GS for Speed and not the 3GC for "compass" or 3GV for "video recording." Speed is the central upgrade here.
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In a Nutshell The iPhone 3GS is the best all-around Smartphone available....considering all the Pain Apple has taken to ensure maximum features to add to the Pretty Good iPhone 3G, and it pretty much makes up for the Extra 100$.:hap2:
Read full review at : Gizmodo - iPhone 3GS Review - Iphone 3gs review