Using it is extremely simple. You put some water into the flashlight-sized device, shake it, then push a button on the side. That process mixes in the purifying salts and triggers the electrical current (from two lithium batteries) that converts the saltwater mixture into the oxidants. Then, you pour this stuff into your water. Paper test strips that come with the MIOX are dipped into your water to tell you when it's safe to drink.
Is there a downside? Alas, yes. The purifying solution takes time to work. That ranges from 15 minutes to kill viruses and bacteria, 30 minutes for giardia, and up to four hours for cryptosporidia. So you can't just stop at a stream, drink, and be on your way. But, you can use a water filter such as MSR's MiniWorks EX ($80) to filter out those water-borne nasties, then use the MIOX to take care of the germs. That would give you safe water in 15 minutes, which isn't bad. And there's no funny taste.