Personally, to answer the original question, I would deffinitely have to say building one is by far:
- The most fun - I mean you get to actually see it coming togfether, and you can take your time, or you can get it done in a few hours!
- Cheapest - This is by far the most attractive part of building it yourself, due to most companies giving you a good deal, but then charging you a fortune to either make it, deliver it, or just by adding loads of tax on top of the price.
- Most rewarding - You actually see it coming together, and you feel at the end of it that it is YOUR computer, and no-one else has exactly the same as you.
- Gives you a better understanding - This is another add on really to the cheap part, primarily down to the fact that if something goes wrong inside, you will have an idea of what it is. This way you won't get ripped off by a company when you ask them what's wrong
- You can brag - Easily one thing you want to do when you show your mates your new computer, especially when you've got like 7800GTX or a new dual core processor, or even just the simple LED fans.
I just recently built my computer (like last week) with the spec below, and I can assure you, for my first build, it was easy, fun, cheap and very, very rewarding!
But if you're intent on buying an already built computer, please go somewhere where they either know what they're doing, or go somewhere where you can get what parts you want, and not get ripped off with parts you don't need.
ONE THING YOU MUST NOT DO IS USE DELL!! AT ALL, NEVER, DON'T EVEN THINK ABOUT IT! Seriously, they are rubbish at building and they overprice on all models except the XPS GEN 5 (but then for the price of one of them, you could have twice the computer if you built it yourself
)
Hope that helps