I believe it runs fine: the 2009 iPhone 3GS processor is slower than the 2010 iPhone 4, but parses/pushes 1/4 the amount of pixels. From experience, while neither of them are particularly fast at CPU-bound operations, the iPhone 3GS and iPhone 4 are plenty fast enough for daily use on iOS 5 and are still quite smooth for things like scrolling and animations.
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Every iPhone 3GS that's been sold for the last 6 months runs iOS 5 and I believe the majority of pre-iOS 5 3GS users have upgraded to it.
And how does it perform? Does it have all the same features?
I believe it runs fine: the 2009 iPhone 3GS processor is slower than the 2010 iPhone 4, but parses/pushes 1/4 the amount of pixels. From experience, while neither of them are particularly fast at CPU-bound operations, the iPhone 3GS and iPhone 4 are plenty fast enough for daily use on iOS 5 and are still quite smooth for things like scrolling and animations.
Mine has all the features (sans siri) and works fine.