I agree 100% with Roger about the Steripen. They work well. I have the older Adventurer model and am waiting for it to break or for a lamp replacement. It still keeps lighting. I recommend his articles for a somewhat more technical look.
Since they are nearly instantanious, they save weight. You no longer have to carry extra water while chemicals "cook". 'Corse, in a desert, or a dry stretch, you need some extra, based on water availibility. Generally, most places I hike are 3-4 hours max between water sources. Soo, I drop a 1/2L bottle. "A pint is a pound..."
It is recommended carrying a spare set of batteries. Expensive and a bit heavy, but worth the safety. Flip one battery over to prevent current losses. The rechargables don't give the required current and are heavier. So, spend the $12 on a set of lithium's. Well worth it.