For starters, don't be afraid of their QC or CS. Dell's customer service is pretty top notch (from my experience, some people will tell you otherwise). If there's a verifiable problem with any non-custom component, they'll have a replacement at your doorstep in about 48 hours. Custom components may require service, this naturally depends on your location.
As far as comfort goes, nothing beats the Alienware's rubberized frame. It's extremely comfortable and never gets hot or sticky. All the heat is exhausted well out the back and can be used comfortably on a lap while gaming. Every other gaming laptop I've ever tried gets very hot on the bottom. Even after a year of very thorough use it has not become discoloured at all.
As far as performance goes, if you need more than a desktop GTX 680 worth of performance than a laptop is not the solution for you. The GTX 680M is an extremely powerful mobility card but it also draws a lot of power. Two of them will put out about as much heat as a desktop 7970 and cost three times as much, keep that in mind.
The only Alienware laptop that can handle SLI is the M18x and it is big. It is not something you're going to want to lug around all day so keep that in mind.
As Pinhedd said,Alienware laptops has a great build quality and one of the best tech supports.
The only laptop that will give you better performance than GTX 680 desktop is the AW M18x(with either 2 680M's or 2 7970M's)