I'd like to build my own but with the pre-built prices now a days it's just not worth my effort. I don't need a whole lot of horse power more RAM than anything.I always build my own. Trying to find good, low-priced laptop parts is a challenge though, especially when places like Dell are practically giving away decent laptops for $299.
I usually change my PC about every 3 years, and spend about $3000 to build a new one. The end result is a PC that would easily cost over 2x-3x that at any custom PC store like http://www.ibuypower.com or http://www.falcon-nw.com/
Luckily I really only need to change my motherboard and processor next year, and possibly bump my RAM from 16gb to 32gb. Everything else (Hard Drive, Sound Card, etc) is all still top-of-the-line, so I don't need to replace it. So I would probably be spending about $750 as opposed to $3000.
As I write this my prebuilt is out for repair.
Under warranty thankfully.