I have one, they're super great astringents for stopping bleeding of small cuts :D sadly they're not very effective on my level of sweat/stink 😭 though they are better than nothing so I use mine as a backup when I've run out of aluminum chlorohydrate antiperspirant. They are indeed cheap if you know where to look, and I've never had one run out (I just drop them and they break 😂)

of note, potassium alum / any alum mineral does actually contain aluminum ions by definition. they are known to be non-irritating to some people who have a reaction to more mainstream antiperspirant ingredients (e.g. aluminum chlorohydrate, aluminum chloride) hence their popularity as an alternative