Count me in as a PBW believer.
I had a hydratube on loan that came back with an unsightly ring of hard water buildup. I tried soaks as long as 24 hours with vinegar, pickling vinegar (7% acetic acid instead of 5%... lol desperation), CLR, Oxyclean, as well as the normal resin removers like ISO and salt, acetone, Simple Green, and Grunge Off Super Soaker. Nothing would touch it. I had already grudgingly accepted that it was a permanent stain when I ordered some PBW.
After a 20 minute soak with hot water and about half a teaspoon of PBW it was reduced to a faint ring. I hit it with another 30 minute soak, same amount, and it was
gone. Thank you, t-dub and everyone else who's shared their experience. What a fantastic addition to a glass cleaning toolkit.
Something for the chemists: A product with similar ingredients (sodium metasilicate)
warns: Do Not Use On Glass.
Seemingly because it can leave a hazy film. Wikipedia suggests that the stuff can actually
turn into glass under the right conditions.