I would agree with Tony's very good points:
It's about pride of having a machine you have built all by yourself. It couldn't be more 'your own'.
It's also about banging your head against a wall when things don't go quite the way you planned...
...but that where the best bit comes in - it's about learning from your mistakes, and then perhaps being able to help others through your experience.
When you do your 'shopping', don't forget all the bits and bobs, like the case, cooling fans, CPU Fan, cables, and that sort of stuff. They all add to the cost a little bit, and what you don't want is the frustration of not having all the bits when you come to start your project. You'll also need to consider which Operating System you are going to use, and how you are going to acquire it. Not every retailer sells OEM OS's, and the ones that do, will usually only sell it with other parts, such as a hard drive. I think Ebuyer do, CCL does, and so does Redstore.