Best user interface I have used.

The best interface I have used is the interface of my second generation iPod touch. Functionality is pretty self evident, the user model is simple, the input mappings are either natural feeling finger motions or explicitly written out and above all it is (still) fun to use.

Clear Input Mappings

The biggest advantage of using a multitouch screen with (almost) no physical input buttons is that mappings between inputs are necessarily limited. If the application designer wants to have the user interact with the interface they must either put a button on the screen that performs a specific purpose or use one of the well established finger gestures that are present throughout the system, such as two finger pinch for zooming or one finger vertical and horizontal swipe for scrolling. This leaves little room for the ambiguous input mappings that can be found in systems with more physical buttons. As I have more recently moved to an Android mobile device this has become more clear, as the functionality of the hardware buttons often changes from application to application and even within the context of the core operating system.

Simple User Model

The simplicity of the user model in the iPod touch is another strong point. By this I mean it is clear what is going on with the system when you are using it. If you press on the button of an application it turns on and you use it. If you press the home button the program goes away and you do not have to worry about whether the application is still running, whether you saved what you were doing, whether you quit it properly. All of that is handled by the application and OS, with the only exception to this function being the music player that can run in the background. Again this is in comparison to my recent Android device, where it is never quite clear what state a program will be in when you return to it. Did it quit when you left it? Is it still running and consuming battery? Did it crash? These questions are simplified in the single application/process at a time user model of the non-multitasking version of iOS.

Fun to Use

Finally one of the things that impressed me the most when I first started using the iPod touch was how fun it was to interact with things with your fingers. This was compounded by the smooth and logical way things moved. I honestly spent hours idly scrolling between the home screens on the device because the movement was very satisfying. The kinematic scrolling introduced to handle scrolling through long lists was similarly satisfying. Scrolling through a long music library looking for a particular song was easy and actually fun, where performing such a task on a traditional ipod or laptop would be a long, imprecise process.