I use collected water from the runoff of my shop roof. An old sponge and towel and still get a spot free car at the end. The time is a little more involved but I think it gets it cleaner. Also rain water due to the natural acidicness of it removes bug splatters really well. The only problem I ever have is my windshield. I gave up long ago and when it is nice and clean from road dirt and grime I simply take out the windex for the streak free shine.
