If your in a hard water area - I.e South/South East of England then your going to have a hardness between 150 ppm - 400 ppm and you will get some bad staining if you dont dry your car off. In my particular area it is 350ppm which is pretty dam bad but have a water softener to combat this. Same principle as the Vessels but this also helps to stop scaling within heating systems, no more staining on worktops/taps etc and lots of savings on cleaning products!
Good thing is though we have the softener plumbed in a way that we have got a 3 taps at the back with two on soft ( hot and cold ) and one still left on hard water for watering the garden. Its great for washing cars