If you are looking for something a little more high-end then you can get with a "big-name" vendor (Gateway, Dell, IBM, HP/Compaq, etc.) then Alienware (www.Alienware.com) can be a good choice. They are a nice fit between the big-names and the boutiques like Voodoo (www.voodoopc.com), Falcon (www.falcon-nw.com), etc. - though in some cases that is a very small space.
The distincive Alienware case looks like it was inspired by Darth Vader's helmet - and they have a special Star Wars model available - but beneath is standard case frame that accepts standard motherboards, power supplies, etc. You can also get PCs with more subtle case designs though they still have the alien head logo on them. The standard underlying case is a big advantage for upgrading later on and a key advantage over the Dell systems that use more Dell-specific components. You can't put a standard ATX motherboard or a standard power supply in a Dell - you can in most Alienwares (the small form-factors are, of course, still a problem).
Alienware doesn't let you mix-and-match brand/model components the way you could if you bought your parts from NewEgg (www.newegg.com) or if you were buying from a place like GamePC (www.gamepc.com). The parts in an Alienware PC match the specs they describe (make/model of processor, type of memory, size/speed of disk drive, etc.) but not a particular brand/model - so if you want a specific model ASUS board or Hitachi drive then Alienware won't work for you. You don't get to pick the parts but the parts are good. I haven't looked under the case of one in awhile but last time I did, even when there was an Alienware sticker on the item, you could tell they were using high-end (Antec, Zalman, Asus, Hitachi, etc.) parts.
One remaining differentiator may be service. Alienware used to only offer mail-in. They tried to use Best Buy as a channel for awhile to expand sales and as a point of service but that fell apart. They certainly have shed their "boutique" roots in that they now target business, government and education...but I don't know what their service options are or how good they deliver on them. Most of the people I know with an Alienware are gamers/artists/developers pretty comfortable fixing things themselves and/or dealing with so-so phone support - support is less a factor. May still be good - I just don't know.
Alienware PCs consistenly do well in reviews in places like MaximumPC (www.maximumpc.com), PC Gamer (www.pcgamer.com) as well as PC Magazine www.pcmag.com). This is sort of like 4-door sedans in that while only one gets the "Editors Choice" in some issue of some magazine year the reality is that there is a group of good choices to be had and that rate well year after year.
Alienware offers desktop CPUs in the laptops. This has the advantage of offering a more powerfull processor in a portable form-factor. That said, even for most high-end purposes, the mobile CPUs are very good choices. Alienware offers those as well but so do Dell, et al. The other consequence of the desktop CPU is that the heat generated takes the "lap" out of "laptop". That is no joke as I've used some of these machines and as fine/fast as they are they certainly are hot and unpleasant to use sitting in the couch - such use may also damage the machine as they are very sensitive to airflow from the side/back/bottom vents. Portable or laptop also becomes questionable after a point due to size as some of these model have the largest screens available, RAID drives, etc. - "luggable" is more accurate....fine to back away to the vacation house but no good for using on a plane.
The boutiques (Voodoo, Falcon, etc.) offer more exotic choices - water cooling (though that is getting more mainstream these days), fancy paint jobs, and cutting (even bleeding) edge components.... all at price point that would re-roof my house rather than go into a PC.
The mix-n-match shops like GamePC give you more control over the components (nice if you have some of your own to re-use) and will do the assembly and burn-in test for your.
The only big-name to compete really with Alienware is Dell with the Dell XPS line. These Dell products do very well in reviews even, grudgingly, by the niche publications for gamers that pre-disposed not to like a big-name company. Dell has also segmented not only products with the XPS line but service - so part of what you get is not only a high-end PC but higher-end service.
If you are looking for a general consumer PC, Alienware isn't in that market so much. At this point it is like buying a TV and there a lots of choices. There also may be factors (ease of service) that make one go retail in this situation as well - Alienware doesn't have company stores nor (since the fall-out with Best Buy) offer their machines at retail.