Good points all, but let me summarize if I may.
Alienware is not junk in a pretty case. They know how to put quality components together in tested configurations for people who don't know how. There is value in that knowledge and assembly. Their cases are pretty, yes, but they are also designed for ease of use and superior cooling.
It's true that if you know what you are doing, you can build your own high end system for less money, but if you are using quality components, you really don't save that much. I'm not talking about numbers, I'm talking about quality components. For example, the difference between a Maxtor HDD and a Seagate HDD (and I really don't want to start a discussion on who makes a better what).
So if your not a hardware genius, and you want a good gaming system with a warranty, you can do a lot worse that an Alienware system.
EDIT: No, I don't own one. I design and build business systems for a living.
Crazy Grandpa