I have been amazed by the goodness of Mr Clean Stain Erasers (the white soft spongy-like things). I use on tub with just enough gentle soap to moisten -- Dr Brommers works, any shampoo works, and they leave things smelling good.

I had used the erasers for years without realizing they worked on soap scum [duh].

ALSO: a clean one used with water will remove tea and coffee stains from a cup LIKE THAT. Better than soaking in bleach, even.