I tried a Steripen Adventurer in the summer of 2008 and returned it to REI after its first trip.

I had a lot of trouble turning the thing on and off because the switch was so stiff. In fact, I had to get my son (who was on the trip) to help me with it. Since I generally backpack solo, this alone was enough to make the Steripen useless for me.

I read the directions extra carefully--even took them with me down to the creek--but the thing kept aborting the cycle during the stirring process. It would take 2 or 3 times to get it to go through the entire process. I was being careful to keep the thing in the water and no, it wasn't the low battery signal.

Another thing I disliked about it was having to carry a Nalgene bottle. The Nalgene and extra batteries pretty much swallowed up any weight savings over a gravity filter.

The third thing I disliked was having to sit by the buggy creek and stir and stir. The Steripen is definitely not suitable for a group (there were 4 of us) unless each person purifies his/her own water.

After a day of this nonsense, I put the Steripen away and used the Katadyn MicroPur tablets I had brought along as backup.