The water treament market is flooded, if you'll forgive me, with abundant options. We all know the general choices: take your chances, UV (e.g. Steripen), pumps and filters, chemical, and boiling. I won't go into the merits and deficits of any one category here, but I would like to talk about one of my favorite pieces of gear in the past couple of trips around the sun: The Katadyn BeFree filter.
I'm out and about in the mountains in a huge variety of circumstances: basecamping in Wilderness with a group I'm working with, where a Guardian filter supplemented with chemical treatment is my prefered water purification method; shorter trips, where I love my SteriPen; and all day, or longer, runs, where I either risk it or take my BeFree. More and more the BeFree is my choice for most of my personal outings.
There's not much to dislike about the unit - it's light and compact, weighing in around 2 oz, and at just under $40 it is relatively inexpensive. It fits into most pack's hip belt pocket, and quite easily into my running vest side pocket.
Using it couldn't be easier - unscrew the top, fill the soft plastic bottle up, screw the top back on, gently sqeeze to either drink directly from the bottle, or to fill another container. The unit holds a liter (they also make a .6 liter version).
Like any filter, you want to start with the cleanest water available. If you do that Katadyn states that the unit can filter up to 1,000 liters. If the filter does get gunked up, which mine has yet to do after probably about 200 liters through it, the unit can be cleaned by filling with clean water and shaking for thirty seconds or so. In my experience, once a filter starts slowing down it's days are numbered and should be replaced. The BeFree has a replacement filter available, but it's not much less money than the complete unit. The BeFree is a .1 micron filter and is 100% pvc and BPA free.