In general, most people think they’re building an everyday app, but they’re not. When the actual use patterns are discovered, most apps will be used every few days or less. Designers have to ask themselves a very hard question: “How often are people really going to use our web application?”.
In our research, we found that the most effective teams were skilled in all five styles, choosing the style that best fit the needs and goals of a project. For example, they might concurrently be involved in deep research on a User-Focused project, while relying on their experience for a Genius designed project, and spend a little time whipping out some one-shot functionality whose results would be Unintended Design.
Since the teams are working with different styles all the time, does it matter? Our research says it does. The teams that produced the best experiences knew these styles well and how to quickly switch between them. They knew when they needed to go whole hog and pull out all the stops for a User-Focused style project, while also knowing when it was important to bang out a quick design, knowing the results would essentially be unintended. Those teams had a rich toolbox of techniques and a solid understanding on how and when to use them.
There is no silver bullet.
We recently published "How people really use the iPhone", an interesting readout of design issues and recommendations for people designing for the iPhone and beyond. Our presentation on this topic at the iPhone Developer's Summit was called the "best presentation of the conference" by several attendees.
Some of the problem exposed applies to other system imho.