Do you purchase your tower computer(s) off the shelf, build it yourself, or have it built for you?

When I think back at all the computers I have owned, if it was a tower, I built it from scratch or rebuilt it from an existing computer. To me it is the only way to go, you get the board and CPU you want, or as much as your wallet allows, you can have extra HD, burner and player, the max amount of memory, extra cooling fans, etc. I can figure out what I want the computer to do, then I shop the parts and put it all together. I feel I get what would have cost me 30% more or better if I purchased it that way off the shelf or had vendor order it that way custom made. The towers you purchased in the past were they off the rack, custom order, or you built it? Maybe you never bought one and always had one kicked down to you?