Building is for someone who wants a hobby. Naw, building is for anyone that doesn't want a crappy computer. Shopping for components doesn't take any longer than shopping for a dell or gateway. You still have to figure what components you want in those and if they are any good or not.
Box suppliers like Dell, Gateway, etc. also do not target the very premium market very well. They typically don't offer the highest tier of video, RAM, HDD setup, etc, unless you get the flagship $6000 PC. Can you say ALIENWARE?
Box suppliers do beat everyone on price for midrange systems, Only if you include all the extra useless software you don't want. As you can see above I matched Gateway's price no problem. I have also beaten Dell's prices when building for a friend.
As for components, you can guarantee a reference design for almost all components. PC makers don't want headaches or design costs, so they order reference boards from Taiwanese manufacturers. reference design doesn't mean quality
You will almost definitely get a working, installed PC out of the box from any large vendor, whether Dell over the web or e-Machines at CompUSA. When you buy parts from the cheapest 6 websites you'll probably run into a couple of headaches getting everything installed and working. If an afternoon playing with hardware and installing an OS is fun for you, it sounds like you found a hobby. yup, this is where the big guys have us beat hands down. they build it and it shows up working. We have to take a couple hours to put ours together.
Warrantee is also very nice. Lots of lower cost mail order components have 90 day warrantees, and you have to track down the manufacturer. It's easy to find brand name components at a similar price point (or the same price point) with one year, or longer warranties (5 year, 10 year and even lifetime). And yes, you do have to deal with the manufacturer, but they have phones and websites and email just like dell.
I personally don't care that much about upgrade paths or the special little features of my hardware, I buy low-mid range PCs that will support whatever high end video and RAM I will add. I haven't upgraded a processor in a PC for myself since I bought a K6-2. If my PC is two or three years old and I need more horsepower I buy a new box and add the upgrades I need. Last summer I got a $499 PC, I added a $250 video card and $100 RAM, and use my old PC for a spare. I might have been able to get all of those components from pricewatch and come close to that price, but it would have taken me a week or two to get everything and another week or two to get everything working, and I already have too many hobbies.Much to your satisfaction, you just caused my head to explode from imaging e-machine with a 7800GT and crucial CAS 2.0 RAM and the absolute abomination that it is.