I've got a 25' 3/8 immersion chiller. My well water is pretty chilly even in the summer (I live on the side of a mountain in Maine!) so as long as I constantly stir, I can drop 10 gallons from boiling to low 60s in 30 minutes.
I've been looking into a plate chiller for awhile, but I think I might just get a pump and rig up a whirlpool setup a la Jamil. I'm sure it'll work quicker, plus I won't have to sit there and stir.