I've had about 8 computers so far. 4 of them I've assembled myself, but the last 3 I have purchased.
My current one is an Alienware Aurora. More on it later
My previous was an Gateway, and the one before that was an HP.
If I can build my own computer, why have I purchased complete ones recently, you ask? It used to be that you could save a TON of money assembling your own computer. It seems to me though, that lately, it does not save as much. Let's take my current computer for example, the Alienware. In a previous post, I said I spent $1,700 on it, and someone posted that I could have put it together for about a thousand dollars less. Let's break that down. By doing some web searches, here's some representative prices:
CPU: Intel(R) Core(TM) i7-2600K CPU @ 3.40GHz: $290
Video card: 1GB GDDR5 AMD Radeon™ HD 6870 $200
Memory: Kingston 4GB Dual Channel DDR3 at 1866MHz: $128
Hard Drive: 1TB, 7200 RPM, 3GB cache: $115
Middle-grade keyboard: $50
Middle-grade mouse: $30
Fairly high-end case: $100 (the alienware case is something special, BTW. Easy access inside, awesome design. One of the things they're famous for, so not exactly apples-to-apples here. But a case design is a secondary consideration to the performance)
875W Power supply: $100
Motherboard: The specs are Alienware branded, so can't really search that, but found similar for $150
Sound card: Sound Blaster® X-Fi™ Xtreme Audio: $35
Windows 7 Home Premium: $80
DVD +-RW drive: $35
So far, that's $1,333.
I may be forgetting some items, but my point is, to build a system like above for $700 I think would be pretty hard.
The other thing that buying a pre-built machine is the support. I've had some fairly good experiences with Dell support (who owns Alienware now) in the past.
A few computers ago, one that I built, I had some trouble with. The sound card didn't work right. What can I do? I called the manufacturer, and the store I purchased my items for? The manufacturer of the sound card said it must be the motherboard's fault, and the motherboard manufacturer said it's the sound card's fault. The store I purchased from let me exchange the card, but that one was having the same problems. They even let me exchange the motherboard. Same thing (man, replacing a motherboard is a pain in the a$$).
I ended up getting a different sound card, and it worked. Part of my point here, is that a prebuilt system has parts that they know WORK TOGETHER. And if there is a problem within the warranty period, you just call the computer manufacturer to get it taken care of. Lots less hassle and investigation.
So, yes, you pay a bit more to buy a prebuilt computer, but I've found in the last few years that the price difference is not as huge as it used to be, and to me, the advantage of having an already-assembled, guaranteed to work right, come right to my door, is worth the extra cost.
And the glowing Alienware alien head on the front is awesome... :)