Ever had a user say "this isn't for me" and realize it was the most useful feedback you'd gotten?

Someone used our product for about five minutes and wrote a short, calm message explaining what they were trying to do and why our approach didn't fit. No feature request, no complaint, no suggestion. Just a clear "not a match." It did more for our direction than a year of "looks cool" and "nice idea." It made us realize we'd been sanding the product down to accommodate people who were never going to stay, while the people who actually needed it got a diluted version. The rejection was honest, and honesty is rarer than praise. How do you tell the difference between a user who's wrong for the product and a user who's telling you the product is wrong?