I agree with Paul. Building a machine yourself is way more exciting and rewarding that buying one of those pre-assembled PCs.
Me too. Not only is it more rewarding but those prepackaged computers have a lot of crap set to run that you never need and just drags performance down. Also I know exactly what components are in my PC and I can replace any one of them with just a quick trip to the computer store. Those brands often have proprietary software that doesn't play well with off the shelf parts.
That said they are fine if you just want to buy something and use it and aren't in to fiddling with your hardware. Never went for that myself. I'm always tweaking and trying new stuff. ;D