I don't think Alienware are anything special to be honest. I think I can make a much better job (and have) of building PC's myself. That way they are more customised - Alienware really aren't that much different from others, they just use performance hardware.
In regards to a laptop, Alienware are slightly better, as you can't really build your own. There are a number of good companies out there however, (depending on your price range of course). A quick google turned up
This.
Just something you should take a look at.
I myself if buying a performance laptop, would look into Voodoo, as they make some real screamers of laptops, and they look really nice too.
Voodoo homepage