I spent a long time looking at the immersion chiller threads and google image searches trying to come up with the best bang-for-my-buck design for a wort chiller. It was built with 3/8" OD soft tubing, and 1/2" hard pipe and fittings.
I'm not going to go into theory for two reasons: one, because that's covered elsewhere on this site in unfathomable depth; and two, 'cause I've been celebrating the success of my handy work and I'm in no condition... In summary, use small diameter tubing (cause it's cheap) in short runs ('cause it heats up fast) throughout the wort (so's to spread around the coolin')
My success? No brew today, but I got anxious and tested the chiller on 5 gallons of water on the stove-top. Using 52 degree F water from the garden hose I went from boiling to 70 degrees in roughly 5 minutes.
I say roughly, because I turned on the hose before I readied the timer on my phone, and the back pressure from the in-line ball valve caused a leak, so I opened it and started chilling to avoid death by wife due to flooded kitchen. There was probably about 15-20 seconds worth of panic before I gave up looking for the timer and realized the microwave has a clock.
I did manage to steady the camera to do the fancy talking for me.