My 4th gen white iPod Touch (shipped with iOS 5) is on iOS 6 now and i find it much smoother than iOS 5, apart from the App Store. The low RAM does get in the way when you switch to other apps and need to wait a few seconds for it to reinitialise, but other than that, it's not a problem. However, i do regret not buying the black version on iOS 4 and comparing the speed to iOS 5.
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My 4th gen white iPod Touch (shipped with iOS 5) is on iOS 6 now and i find it much smoother than iOS 5, apart from the App Store. The low RAM does get in the way when you switch to other apps and need to wait a few seconds for it to reinitialise, but other than that, it's not a problem. However, i do regret not buying the black version on iOS 4 and comparing the speed to iOS 5.