End user testing, product usability, test kitchens. I do it all. Since the bulk of my ware is functional kitchen to table ware, it's mandatory that I try out almost every shape or glaze with my own spoon or meal. That's not to say I try out "yours" but as I come out with a new piece that gets replicated, I do give it an up close and personal.
As a result I have changed glazes in the bottom of a bowl that I just didn't like the feel of, or altered a rim for better pouring. I tend to get draw back to what works in my kitchen, which bowls my kids eat out of the most because they are just the right size.