The SteriPEN Traveler is the lowest priced model offered, coming in at $49.95. The Traveler comes in at a convenient 5.7 ounces with batteries. It's a tad longer than some of the other models at 7.6 inches in length, however SteriPEN has a smaller Mini Traveler model.
All of SteriPEN's models require batteries and Lithium batteries specifically for optimum performance. Being that it is a electronic device does have some drawbacks, such as if the batteries run out (carry extra batteries to combat this) or damage to the light or system. There is a protective cap that covers the bulb for protection. With that always carry two methods of water purification in the field.
The thing that I like most about the SteriPEN Traveler is that it works with a standard water bottle. It can be used with any standard water bottle and those can be scavenged about anywhere due to people littering. It can also be used with standard Nalgene canteens and other water containers.
The Traveler is capable of purifying 200 half liter treatments on the same set of Lithium AA batteries. The bulb life is capable of 3000 treatments. For one person this model will last for months if used every day.
The system is extremely easy to use. Grab some water from a water source. Here I grabbed some from a stream. You may want to use a cloth or handkerchief to filter out small particles. Always submerge your bottle away from the current so "floaties" don't wind up in your bottle. This system will not help with the taste of the water.
Then press the button once for a full liter purification or twice for a half liter treatment. The light will begin to flash green when the system is ready. Then insert the Traveler into the bottle and flip upside down. The Traveler has a rubber gasket to ensure no leakage when flipped upside down. Once the to conductor post hit the water the Traveler will start the treatment process, once the UV light has cut off its ready (taking about a minute). If the system did not treat properly the light will flash red and you will need to do the process over. This could be due to the turbidity of the water or the individual not waiting long enough.
Check them out at http://www.steripen.com/traveler