I had a lot of problems with a Steripen Adventurer and returned it after one trip. First, the switch was so stiff that I couldn't turn it on and off without help--not a good idea when most of my trips are solo. Second, it kept aborting in mid-cycle, forcing me to treat each quart 2-3 times. Third, sitting by a buggy creek stirring and stirring is not my idea of fun. If you're going with a group and you must use the Steripen, then everyone should have their own and treat his/her own water.

When you're looking at the weight, remember the following: A wide-mouth bottle is required, and Nalgene weighs a LOT more than Platypus. Extra batteries are essential, and I wouldn't go out without two extra sets. It's a good idea to take some chlorine dioxide tablets--I've heard enough reports of unreliability that I wouldn't want to trust the Steripen without backup. Add up these essential accessories, and you really haven't saved anything over my 7.5 oz. ULA Amigo Pro gravity filter (unfortunately discontinued). For solo trips, I take just the ClO2 tablets.
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey