Use Open Source or Build Your Own

As I have now been working in IT for the past year as a Software Engineer (specialising in Web Application Development), and been actively building web application for the past eight years, I have begin to wonder if using an open source application and extending it is better or worse than building your own application from the ground up. I know that open source has most of the work done to begin with and I have meet people that will argue until they turn blue in the face that open source is the way to go. I’m a person that is not easily convinced and I find it better to learn how applications work by looking at them in the most basic of forms.