I've been using a Steripen Adventurer for three years now and other than battery problems and problems with my not reading the instructions carefully, it has performed flawlessly. You need GOOD batteries in it; I use Panasonic lithium's in mine. I used it for three weeks in the summer of '08 when I hiked the JMT and had no problems other than my carelessness. I changed batteries halfway down the trail though the ones I started with were still strong. I did carry MicroPur as backup and used it when I was too lazy to fire up the pen. I don't carry it with the batteries installed: for some reason, the batteries don't last very long unless you take them out between uses.

I don't carry it here in Arizona because what water there is, is often pretty murky; I prefer a filter for high-solids water.