I have used steripens on the trail twice, last year, I used a classic steripen which failed after 4 days of use -- it got moisture in the chamber which causes it to fail. In discussions with Steripen, I was informed that their newer models are much more moisture proof than the classic. I went on a 15 day hike in SEKI this year with the steripen adventurer, and it worked just superbly. No problems. We got resupplied after day 7, and only then did I swap batteries. I averaged minimally 4 liters of water purification a day on the hike (some days 5 liters).
PS - my friend who hiked both trips with me had same gear, his steripen classic did not fail last year, thank goodness, as that was what we depended upon for the last 5 days of the trail last year. It rained 8 out of 9 days last year on the trail too (this year it did not rain even once).