"Moving from a set of mechanical controls, such as dials, sliders, and buttons, to a graphical touchscreen is often a developer's dream come true. The developer is no longer limited to a fixed family of settings limited by the number of dials on the front panel.
Menus allow infinite user-configurable options. Scrolling means that you can provide endless lists of data. The GUI provides a window into the internals of the device that was previously unthinkable.
Many designers get carried away with the new toy and foist upon users far more complexity and data than they want or need. Making some of the controls mechanical rather than graphical can have major usability advantages, which we will explore in this article.
While a GUI has many advantages over custom displays, it's important to note a couple of the disadvantages. A GUI allows a number of different controls on the screen, but they all have the same tactile feel when making an input. If the input is via a touchscreen, they all feel flat. If the input is via a trackball, the same roll-and-click motions are used to manipulate any of them. It's possible to build some tactile feedback into a touchscreen, but it's limited.
In contrast, a throttle controlling the speed of an aircraft will be physically larger and have a heavier feel than the volume dial for the aircraft radio. These differences communicate the significance of the action to the user. Imagine trying to drive your car with a mouse and screen as the only inputs and you'll get an idea of how the feeling of control can be lost. The car is quite an extreme example. In a car, it's important to be able to perform one action with each hand at the same time, such as steering and changing gears.
Operating two controls simultaneously on a single GUI is generally not possible. Even when the technical challenges are overcome with hardware capable of interpreting multiple touches, you still face the trickier problem that if two independent controls are on the screen, you're likely to cover information with your hand while performing one action, but that information needs to be visible to perform the second action.
The iPhone allows multiple touches, but they're generally connected to the same action--for example two fingers are used to shrink a single picture. The gesture is multitouch, but it's still a single gesture.
Custom controls can be laid out in positions that fit with the function performed. If there is an eject button on a DVD, it's intuitive to place it beside the slot through which the disk will emerge. If a GUI is the only means of controlling the device, all controls must appear on that display, which means that those controls are further from the related hardware.
Another disadvantage of the GUI is that space doesn't generally permit the important controls to be permanently visible. This may not be acceptable if the device is used in a situation where the user needs emergency access to certain controls, or where some monitored information must always be visible.
Custom controls can scale the user's action or exaggerate the process being controlled. A bigger steering wheel allows a driver finer control over the angle of the front wheels. The da Vinci surgical robot enables the surgeon to move his hand several centimeters to control the robot, which only moves several millimeters, allowing the surgeon a level of control that he could not achieve if he were holding the scalpel himself." (Continued via Embedded.com, Niall Murphy) [Usability Resources]