I don't think that's true, when I factor in my assumed costs you can't build a PC that's twice as good. you might be able to pick up a better video card for the same price, but when you factor in the fact that the AW is small, and has excellent customer service and warranty terms. You can't have both of those things and have it cost half as much.
Alienwares are good computers, the new ones at least, and they are competitively priced. the people who say they are way overpriced have usually never owned one, or don't understand how to build a comparable computer.
Yes, you can build an i5 computer with a GTX 660 for 30% less an than an alienware, but that doesn't factor in that Windows costs money and it also doesn't factor in that you'd have to use a cheap crappy case and a power supply, which is be HUGE.