I used to use these when I first started backpacking…
Price Paid: ?
I used to use these when I first started backpacking 20 years ago. They must be effective since NOLS uses them, but they make the water taste like iodine (no surprise). Then my husband and I met another couple while we were backpacking who were using a First Need water filter (there weren't too many others on the market at that time). They kindly let us filter some water for ourselves. That early water filter basically required two people to use because it didn't attach directly to any container (it used the filter made for camper vans). But we were so happy to find a way to get good tasting water that we bought one for ourselves and I have never used these pills since then.
Only advantage over water filters is compact size and minimal weight, but it definitely makes the taste of the water rather unpleasant.