My experience with the Steripen is limited to 2-3 trips, but it's similar to what Pika and Barry report. I have the Adventurer Opti model which has an optical water sensor instead of electrodes. This avoids the problem of some of the older models which couldn't detect being immersed in water if the dissolved solids were too low to detect using electric current.

Except, when I first purchased it and tested it at home, the UV light seemed dimmer than what I expected from photos and videos even with the room lights very dim. That's very subjective, but from my experience with photography, I could tell that the UV light was 1-2 stops dimmer than it should be. Then, someone on another forum reported noticing this and sending it to the manufacturer for testing. It had a bad bulb which was not emitting enough UV light to perform its intended function! The manufacturer sent a replacement unit. After reading that, I took it on a trip anyway because it was a winter trip in Michigan's UP which would be mostly 20F and below, and that's not the place for my Sawyer Squeeze filter. I didn't get sick, except for a mild case of diarrhea which may have been Giardia, or unrelated to the water I drank. I sometimes drink unfiltered water anyway, so no big deal. I did stay hydrated mostly by eating snow because access to flowing water was infrequent due to 4 feet of snow.

Anyway, after the trip, I sent my Steripen Adventurer in for testing, and it had a defective bulb too. I was sent a new unit with a bulb which is clearly brighter. A dimmer than normal bulb is something one of the quality assurance testers would've spotted easily. It's not clear if the bulb was made defective, or if it became that way during shipping. I like the Steripen, but I also have plenty of doubts. I usually hike in places where I drink carefully selected water unfiltered/untreated sometimes, and I just view the Steripen as optional insurance in case I need to drink from a more questionable source.