My group had a steri-pen on our most recent trip. The guy that owned it was a nut about it (he's sort of a germ-a-phobe) and doesn't drink even the purest of water up there. Anyway, its pretty nice. It does a 32 oz. Nalgene in 90 seconds, which is pretty impressive.  The only problem I saw with it was battery life. I think part of the problem was that it was being put away "wet" and the contacts that make it work were actually working while inside the case. If not, the battery life is pretty low (think about 15-20 sterilizations.) For a solo thats no big deal, but for a group of 4 that was not cutting it. The batteries it used were like camera batteries, but newer models may feature AA or AAA power.   Also, watch the following   (You need to Login or Register;  :)    His little theme song made us all laugh for a week every time we used it!