A hallway usability test is where you grab the next person that passes by in the hallway and force them to try to use the code you just wrote. If you do this to five people, you will learn 95% of what there is to learn about usability problems in your code.
Good user interface design is not as hard as you would think, and it’s crucial if you want customers to love and buy your product.
But the most important thing about user interfaces is that if you show your program to a handful of people, (in fact, five or six is enough) you will quickly discover the biggest problems people are having. Even if your UI design skills are lacking, as long as you force yourself to do hallway usability tests, which cost nothing, your UI will be much, much better.