I built my own PC about five years ago. It was a lot of fun, and I learned a lot about hardware, which for some reason just never clicked for me until I was installing my own memory sticks, etc. It was totally worth it, though it did cost around $450 (also non-gaming PC, and not counting screen, keyboard, etc). I installed Linux on it just to learn, but this fell by the wayside over time. I still have the computer, but it's kind of dead weight. At least it still looks cool.
I'm a software developer, and I do some programming outside of work. I've been more in the habit of buying refurbished Dell laptops from before Dell ditched the docking port. Love those docking ports. I have it hooked up to a large monitor and I have a separate wireless keyboard and mouse. So I just plug the laptop into the docking port and my workstation "powers on". That won't work for the newer laptops, but the idea has stuck around, so there are many similar connectors for various computers that do the same thing.
My biggest gripe with a previous refurbished laptop was that I did not pay attention to the screen resolution the first time, so I bought an "upgrade" refurb and love it. Seeing Visual Studio in crisp, clear colors makes me happy, and some of the later models are gorgeous. But if you aren't buying a laptop, then screen resolution won't even be a consideration :). Usually the specs are perfectly fine on refurbs, just know what you want and buy the best condition you can for the cheapest price. Considering these laptops used to cost $2000, I feel paying $350-400 for one that's in great shape is worth it. Mass produced business computers often have available spare parts on ebay, too, though OEM screens can be pricey.
Just some thoughts :)
(oh, also, I'm amazed at how freaking loud the fans that go into custom computers can be. Most of them drive me insane, I had to drop $40 for a processor cooling fan that was silent and didn't create an annoying whirring sound. Granted, I was working with a slim build so my options were limited, but the fans in pre-built computers never seem to have those issues, even though they are dirt cheap as per mass production. Sigh)