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I recently upgraded from a iPhone 3GS to an iPhone 4. Wow, the differences were more noticeable than I thought they would be! The first thing I noticed and was in awe of for about a week is the retina display. It really is a major improvement at how good the screen it. The pixels along the edge of icons and images seem virtually invisible on the iPhone 4. Also, I noticed that there is a faster response, mostly noticeable when opening new apps. It seems to handle more apps being open at once better as well. Another thing I noticed was the camera. The iPhone 4 has better megapixels in its camera and takes HD photos. The quality of photos is noticeably better!
So, if you are thinking about upgrading, anything higher than a 3GS is worth it!
Image Source: http://en.wikipedia.org/wiki/IPhone4