What is Skeuomorphic Design and When to Use It
Design is full of trends and rebellions against those trends. Some technique, element or outlook becomes fashionable, then a backlash forms seemingly out of nowhere proscribing it. I find it difficult to keep up on trends, and I try to base my process on my own experience rather than the mode of the times. It's hard enough coming up with the best design for your requirements without ruling anything out ex facie . However, there are some popular design trends that just don't suit the needs they're commonly applied to, and skeuomorphs are a good example of this.
What are Skeuomorphs?
Skeuomorphs are design elements that serve no purpose, but stick around because they used to be useful. Software products that do something analogous to an older physical product often contain skeuomorphs: for example, circular knobs to change the volume in a music player, or pages that have to be flipped in a document reader. You don't need to flip knobs or turn pages in a digital world, and it's quite a bit of extra effort to implement these special controls.
While cute, these elements transfer the limitations of the analog world to the digital one, where they're unnecessary and sometimes even counterproductive. Don't make me turn pages for no reason!
Why use them?
So why use them? Apple uses skeuomorphs in apps like iCal, Find My Friends, even on their standard virtual keyboard. They claim these artifacts evoke an emotional response in users and immediately orients them to how to use a product.
While it is usually difficult to argue with Apple's design rationale, I'm suspicious of this argument simply because I see no research to support it, and the history of user interface design provides plenty of anecdotal examples to the contrary1.
I suppose the real reason for skeuomorphism on first-party iOS applications is simply that they look really, really cool, and even if they don't actually make an application more intuitive to use, they make it more approachable. These elements don't contribute to usability, they contribute to marketability. In other words, they help inform you about an app does rather than how to do it.
But I can think of at least one case when the use of skeuomorphism adds genuine value to a user's experience: aiding in the transition between a physical process and an analogous digital process.
For example, a fine artist used to blending colors on a palette may not at first understand the discrete " RGB color picker" model that is the standard in graphic applications like Photoshop. Instead, they may want to select primary colors (or a set of predefined colors mapping to common paints) and mix them together to get the hue they're looking for. This works at first, but it's inefficient and unnecessary, and as they become familiar with the application they'll probably outgrow it. So, why not let them blend colors together on a palette, but also let them select that color, browse through variations of it (a slightly warmer tone, a slightly brighter one, etc.) and then save the color for later use. In other words, use skeuomorphs to help the user approach the application, but do not limit functionality according to any artificial constraints.
Note that the transition between analog and digital processes could go both ways. You could also use skeuomorphic design to help people on a computer to understand how to use physical tools. For example, on a GPS-enabled phone, you can easily locate North with (or even without) the push of a button. On a manual compass, it's slightly more involved -- let the magnetic arrow settle, align directions on the housing to the arrow, and change your orientation relative to the compass. But how about a GPS-enabled application that teaches you how to use a manual compass by retaining the artificial limitations and forcing them into the interface?
I'm skeptical of the use of skeuomorphic design when it is claimed to benefit the usability of a design. I generally believe skeuomorphs serve a mostly decorative purpose and while that's not necessarily an argument against them, form should always follow function rather than dictate it.
1I'd even wager to guess that the trend towards skeuomorphism is cyclical, rising in prominence as new platforms emerge, and waning as these platforms become more familiar. See for example early desktop computer interfaces, which ultimately resolved into the WIMP model, then early web site designs, from which emerged the standard web site interface we're all pretty familiar with. No doubt the standard interface for touch-based mobile applications is still being shaken out.