Keeping this in mind, you need to have a reliable back end, and a functional, presentable front end. The GUI doesn't have to be anything spectacular, just well organized so things make sense. Take this website for example - nothing fancy about this GUI. However, do spend some time making it organized and easy to use since that's what everyone will see. Worst case scenario, hire a designer from a local community college or elance.
Here's some general tips to make your GUI more presentable: 1. Form fields should line up, and generally they should be the same size. Don't have some indented and others not. 2. Try to keep your forms balanced across the page/screen. Scrolling down forever sucks, so distribute placement evenly. Similarly, horizontal scrolling also sucks so avoid that as well. 3. Try to keep user inputs to a minimum. People are lazy. 4. Use colors that work together. When in doubt, leave it black and white. Check out http://www.colorschemer.com/online.html 5. Use colors as indicators. Green = success, red = failure 6. Be consistent.
If you want to read an awesome book about designing GUIs try Smashing Magazine's book: www.smashingbook.com
How important a GUI is depends on the nature of the program. A significant chunk of the world's businesses depend on apps that have no GUI while more or less everything consumer-facing has a GUI.
See this for an example of UI/UX importance: http://ipadtest.wordpress.com/2010/04/20/when-will-lenovo-an...