If you already have rings around the toilet bowl, a couple of popular types of products may just disappoint. Bleach and bleach-containing cleansers often make the stains worse or do practically nothing at all depending on the mineral content of your water. Something in the chemical reaction can actually make the stain darker of just completely impenetrable to the power of bleach.
Some soapy, detergent-based commercial cleaners don't do much more than roll off of the stains and dissolve in the water itself, doing nothing to remove the hard water stains or buildup. Even abrasive powders can be a letdown if they aren't backed up with a lot of hard and long scrubbing, and scouring pads or rough brushes can damage toilet surfaces over time.
Several products do work well because of their chemical concentrations, and these usually are marketed as lime or hard-water removing cleansers, but they don't work for everyone. Even commercial-grade cleansers with bases made from an acid such as sulfuric acid either work really well or not so great depending on the water make up. And, a number of people have sensitivities or bad reactions to strong chemicals.
Thankfully, there are a few more options for seeing the porcelain beneath your calcium and magnesium.