IMO they are no cheaper than alienware and I am in love with the looks of AW systems.
Dell will also let you haggle quite a bit if you call in to their order assistance number. Some reps will give great discounts, some won't. If you get a nice quote, have them email it to you. That way you can call right back and haggle with other reps until you are certain you've got a great deal.
I just saved over $200 on my new m17x R3, which brought the base price down to under $1300. Upgraded to the 2720qm and accidental warranty and ended up paying about what base pricing is. Knowing that if the dog chews it to pieces, I spill a beer on it or if my girl drops it and it will be fixed next day or be replaced is great peace of mind.