I tried this and the soap scum came right off. I'm completely impressed and no coughing from all the chemicals in cleaners. Never buying shower cleaner again. 1 cup each - warm white vinegar (microwave 2 min) + 1c Original Dawn. Spray, leave for 15 min, rinse.
Tip: Once a week, toss in a few frozen cubes of white vinegar and turn on your disposal. The vinegar will get in all the moving nooks and crannies that are otherwise hard to get to and the ice will sharpen your blades.