My mom suggested I use a pumice stone, and.... oh my freaking goodness!! Brendan came home and asked when I had installed a new toilet. It looks so good!!!
So if you have hard water stains and you're trying to get them off of porcelain, check the cleaning aisle of your local megamart for cleaning pumice! I purchased mine at Wal-Mart; it was on the dish detergent, steel wool, etc aisle. It does take a bit of scrubbing, but with an utterly 'oh-my-god-that's-revolting' level of buildup, it took maybe 30 minutes. With a lesser stain, took 2-3.
.... yes. I'm excited about cleaning product. My life sucks. You may laugh now.