Tell you what.
I haven't owned Alienware for Two reasons:
Price: Everything costs way too much. Using very similar, if not better components, I can build their $2500 machine for less than $1500.
Internal Components: Unless you specify, you get the cheap parts. Budget RAM, low profile cards, etc. Even if you specify, They stop at certain speeds. You want faster RAM? You buy it separately, then.
as far as the internal components go, i think they use very good equipment. asus mobo's, evga graphics cards (not sure what they use for ati), elpida ram (i had really good luck oc'ing this ram 1333mhz to 1600mhz no sweat), decent seagate hd (even though they get bad feedback from some, mine was fine), re-branded psu's that are actually well-built. really, the only beef that can be made about their equipment is their cases are far from well-designed. they have very poor air flow (at least it is with my p2 chassis).
i am not trying to start an argument with anyone here. just please keep the bashing out of it. there is no question a computer can be built for much less than alienware charges. it all comes down to personal preference and budget.