Updated 20-Sep-2023
Usability is a complex phenomena, especially when it comes to mobile. Hardware, operating system, application ecosystem, all play a role. I've just updated an old Kindle Fire to Android Jellybean. This experience forced me to compare iOS on an iPad 2 with Android Jellybean on a Kindle Fire. Obviously the screens are different sizes. Nevertheless it is possible to compare the experience of trying to do things on a touch-based interface with comparable applications. Since the Kindle Fire doesn't have a camera or even a microphone or 3G, things like network speed, calls, or camera hardware/software is not a part of this review.
The thing is, iOS is not all that great to use, even with the 6.0.1 update. Android on the other hand has really progressed, quite dramatically over the past year. I've used Froyo and Gingerbread (via CyanogenMod) on an HTC Desire (Bravo) phone for 18 months, and now I've been running Ice Cream Sandwich on an HTC One S for the past two months. Installing a Jellybean rom on the Kindle Fire (which was previously running Gingerbread via CM7) is an eye opener.
Speed and Stability
Both operating systems are relatively stable and speedy, though I have to say the Android system has gotten a lot faster and the performance on the old Kindle Fire (even without any overclocking) is surprising.
Hardware is not the Issue
Neither the iPad 2 nor the Kindle Fire 1 are very powerful, and both have relatively small storage (16gb and 8gb respectively). I've run up against the limits with certain apps (especially games) and trying to get too much music synced on these. So the real comparison I want to point out is the actual operating system and the so-called application ecosystem. What this really means is what apps are available and how do they compare.
App Support on iOS and Android
Some apps are quite different from one platform to another, usually different in a bad way. For example, the Evernote app on iOS is horrible, whereas the Android version is great. (A recent update on iOS made the app even worse, so much for that development team.) A key feature of Google Drive, namely the ability to edit a spreadsheet is not supported in the iOS app. Then there are some apps which are only available on one platform, though this has really narrowed in the past year. Facebook Pages is a great app only found on iOS, as is GarageBand. Sony Reader did not have an iOS app for the longest time, but they remedied that within the past month or so. And some games, such as my latest favorite Autumn Dynasty, is still only on iOS, though Bard's Tale, Osmos, and Age of Conquest are on both. (Age of Conquest works much better on the Android platform, and it also, strangely enough, has a smarter AI.)
Widgets, Launchers, Keyboards
The things that really make a difference between the platforms is widgets, launchers, and keyboard configurability of the Android platform. First the keyboard. Apple won't allow any third party keyboards. And move the cursor within a given word is seemingly impossible. No swiping, etc. Of course jailbreaking the device is always a possibility, however Android supports the use of these things without any jailbreaking. Yes, I had to root the Kindle and install the Jellybean rom, but my HTC One S does not need any rooting to configure multiple different 3rd party keyboards which support swiping as well as Southeast Asian languages (swiping in Thai with TSwipe Pro; Khmer, Lao, Myanmar language support with MultiLing). Widgets and Launchers make a real difference in customizability of the interface for Android. IOS doesn't have anything like that (though again, jailbreaking does offer a variety of customizability options). The problem is for the average user this is simply not an option. Many people don't want to void their warranty and most simply aren't interested in doing so, or just don't want to invest the time needed to develop the technical skills needed to understand what to do.
Conclusion Android beats iOS
On older, underpowered hardware, comparing just the application ecosystem, Android more or less matches iOS. Comparing the ease of use and customizability of the interface, Android beats iOS. Now, some people might still prefer iOS and Apple mobile products. That is fine, but it is really a matter of taste and not fact. De gustibus non disputandum est.
Addendum - OSX beats Windows (and Linux)
Note that I still love OSX and it still (with its flaws) beats Windows and Linux for usability, though this too is more recent than some people might think. The application ecosystem for OSX essentially matched Windows a few years ago. While certainly there are more applications available on Windows, for my purposes as a power user who needs at least comparable application performance on both platforms, has found it on OSX. Again, the app ecosystem, interface customizability, speed and stability are the main issues.