How important is it to use distilled water in a radiator?

[Seanz0rz]

i always use distilled (thought i dont have to worry about that with an air cooled VW!)

even filtered water can be high in calcium and other minerals which build up in the coolant passages.

i wouldnt use anything but distilled unless it was an emergency. thats just my personal feeling on the subject.

[Crinale]

iv heard that distilled is best, but soft water is a close second... so i use soft water in my vehicles... (since my parents house has good soft water)