Android may have a slow upgrade rate, but iOS is far from that. Data from an iOS app developer shows that 61% of iOS users are using the latest operating system update which is iOS 5.1. Data is not universal, it's, rather, based on statistic from iPhone/iPad compatible app Audiobooks.
Audiobooks is one of the most downloaded iOS apps with nearly 100,000 downloads per week and so it is a good app to base estimations on. The table above was based on the users who downloaded the app, it was created by David Smith, the creator of Audiobooks.
As of 14 days after the release of the iOS 5.1 update, 61% of the downloads were coming from users who were under the new operating system. Only 18% were using the previous update, iOS 5.0, with 19% under iOS 4.x, those are probably users of older iPhone and iPod Touch devices which are not eligible for an iOS 5.0 update.
If the number of users using iOS 5.x are counted, the outcome looks even more impressed with 77% of all users using the most recent operating systems. Although the data above is from only one app, it's a pretty good estimation of how well received are operating system upgrades for iOS users.