According to [The Wall Street Journal](http://blogs.wsj.com/digits/2010/07/28/apple-investigates-reports-of-problems-with-ios4-on-iphone-3g/), Apple is investigating reports that iOS 4 causes problems and slowness for users of iPhone 3G. So there might be hope for iPhone 3G users who wants to run the latest operating system on their 2-year old hardware. Let’s hope Apple can fix this issue for the next iOS 4.1 release which is expected within weeks as Apple has just seeded iOS 4.1 beta 2 to its developers this Tuesday.
Even though [not all iOS 4 features are available on iPhone 3G](http://sanziro.com/2010/06/ios-4-missing-features-in-iphone-3g-and-second-generation-ipod-touch.html), there are some reasons why you might want to do so: folder, unified inbox, iBooks, enhanced location service privacy, automatic spellcheck, wireless notes sync etc. Another obvious reason is to run iOS4 only apps or apps with iOS 4 only features such as local notifications and calendar integration. Overtime we expect iOS 4 only apps to increase.
For now, we recommend not to upgrade to iOS 4 if you are using iPhone 3G.