It actually works!
After trying multiple different products with little or no results, I was about ready to just replace my windows (25 years of hard water stains).  I saw a youtube video of this stuff being used with a hand sander and microfiber towel so I decided to give it one more try.  It worked amazingly well.  My glass looks new again and it only took about 5 minutes per window.
Rating: 5 stars