The Great Interweb is filled with sites about designing a product that is usable as well as sites about designing a product that is useful. If you google “usable vs. useful” you’ll even find a lot of great conversation debating the two, but I’ve never really internalized the difference.

My personal moment of clarity came with I picked up my wife’s iPhone.

With its familiar interface, fluid navigation, and multi-touch gestures, I can tell you right now that her iPhone was instantly usable. But after just a few minutes of frustration, I can also tell you right now that her iPhone was completely useless!

…for me.

Using her phone I couldn’t get to my email, my personal calendar, and and I couldn’t even tune my guitar with my favorite guitar tuner app!

That experience was a very eye-opening for me and made me realize that regardless of how shiny the presentation is, and how easy the navigation of a product is, if it is not relevant to me personally, the product is not worth my time.

That made me think: Is it more important to be useful than usable? Can you have one without the other? Also, why do I love products that are not shiny? Craigslist, for example…or my Library app? Neither are examples of awesome visual design, but they are so relevant for me I don’t really care.

That led me to think a bit deeper: Is it an individual personality/generational/character thing that determines which user experience characteristic is most important to a user? If a person is more concerned with the kind of car they drive or other external appearances, does that affect their opinion of a products outward appearance regardless of how useful it is?

How about you? What is most critical for you to have a great user experience?