When are GUIs important?
slindstr: It all depends on who's using your software.  I work on both the front and back end sides of a product at work.  Generally if there is bug it's pretty easy for most to determine if it's the front or back end that messed up, and yet the customer blames EVERYTHING on the front end.  It's not their fault either - they just aren't aware of our architecture, and probably never stopped to think about it, nor should they have to.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