You've probably seen the reviews of the iPhone 4. All things considered, it is a beautiful piece of hardware. The screen, with its tiny pixels and rich contrast and colour depth, looks like backlit slide film or luminous magazine print, and the build quality of the unit feels solid in a way that few mobile phones do. Meanwhile, iOS 4 is very responsive on it. There may or may not be an issue with antenna sensitivity and reception (some users have reported their iPhones losing bars of signal when they touch the steel frame, which doubles as the antenna, and its signal reception fared worse than that of a 3G data card, the only thing I could compare it to on the Three network), though it seems to get acceptable signal so far. Meanwhile, the (rear) camera seems to be a great improvement over the previous one.
I have transferred my data from my old iPhone 3G (one I bought on eBay earlier this year as a stopgap device) to the new phone and played with it a little, though, so far, I'm pleased with it. It'll probably become even more impressive as apps start appearing that take full advantage of its capabilities.
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