We backpacked through Peru for three weeks with this most sensational and mind boggling device. We took water from taps in almost every part of the country and treated it with the steri pen. The 4 of us not once got sick. We even treated water from a well on Machu Pichu! The Steripen is extremely light, portable, easy to use, and simply cool. We got some funny looks as we treated the water in restaurants, but it was absolutely worth it as many of the locals dont even drink the tap water. The money saved on water alone paid for nearly half the device's cost.
I could not recommend this product any higher and will be using it/other of steripen's products as a result of this most impressive product.

I evaluated a pilot Opti unit for Steripen. It was used for 23 days on the John Muir Trail (JMT) last July/August, 2009. It worked just fine. It was less temperamental, by far, than the traditional Adventurer that I own. I did use the flashlight features a couple of times, but principally it was used for sterilizing water. It is also an improvement in visibility in the bright sun but still not as visible as I would like it, one has to create a shadow/shade setup to clearly see that the unit is operating while stirring if done in bright light.
We used for our sterilizing container the 1.9 oz ZipLock Twist 'n Loc® Quart Container. Steripen's support said such a container would work (they did express a caveat about durability in sunlight over time, as I posted their response elsewhere).
One of the other hikers had the Journey Steripen and we were somewhat jealous of the more affirming visual feedback of success, the one downside of the Adventurer or the Adventurer Opti, is that if you don't keep your eye on the unit and have artificial shade setup for watching it, you won't realize if it operated successfully - but with the more clearly visible smiley face symbol on the Journey, it's not so much a problem with it. It's too bad there can't be a separate LED light for "successful" and one for "failure" that stays lit for 10-20 seconds and doesn't require shade to see it.
I like the fact the Adventurer is slimmer and weighs 1 oz less than the Journey. It definitely is an improvement over the older Adventurer. The flashlight feature is nice in a pinch too.

This was my only way to treat water while working in remote regions of Africa. This is considerably easier then my previous water filtration system. It is small, lightweight, durable, works fast and is easy to use. It didn't alter the taste of the water but it made the water safe to drink. I drank lots of treated mountain stream water with no ill effects (some of my colleagues didn't fair so well). I wouldn't consider traveling to an underdeveloped region without this unit.