Who has built their own computer?

I'm into computers a lot, spend most of my time on them, but, honestly, I haven't tried this.  In fact, I know little about computer repair - though, I did study for the repair-person test once.  Well, it gave me a good grounding on terminology.  You ask me stuff like, "What's DDR?" and I won't answer East Germany, lol.

Anyhow, how would you compare built systems to what you buy in the store?  Also, how challenging was it the work?  How expensive was it?  Where did you get the parts?