I've used the prior Adventure model for about 20 days of backcountry use. Its worked very well for me. I upgraded to the new Opti model based the improvements to battery drain while stored and the optical sensor. The activation button was so hard to press, I couldnt tell if I successfully clicked it once or twice, which makes a difference since this is how you set a half or full liter dosage. With the prior model, the click feel of the button was easy to notice. Had to return this, hopefully this will be addressed, but based on other reviews here others seem to have had the same experience. I'd give this five stars and would have kept it if not for the button issue.

This filter is Great and would of gotten 5 stars if the button to turn the pen on was easier to push. Turing this on with gloves is almost impossible.
Ive used this twice now, first in Mexico on there municipal water and second in the grand canyon on a 5 day trip on the escalante route. in Mexico i tried to get water from multiple sources because i figured if it could beat montezuma's revenge i knew it worked. 5 liters of water later i had no problems. In the grand canyon the performance was even better. when my friends with their Katadyn Hikers could not use their filters because it was below freezing the steripen worked flawlessly

This product would be great but it is extremely difficult to get the activation button to work. I took mine back to REI and had it replaced twice but the same problem occured. I contacted Steripen and they say it should not be so difficult to turn on but both of the ones I got were nearly impossible. It literally took squeezing with two hands to get it to turn on. In a survival situation this would not be a good product to have expecially if you are injured, weak or sick. I cannot recommend this product.