"I recently got back into doing some backpacking/camping after years of only car camping. One of the items I added to my gear was a water filter. I bought the Katadyn Exstream, which is a 26 ounce drink bottle (21 ounces with the filter) that fit perfectly into my fanny pack. It also has a ring that allows it to be clipped to your pack using a carabiner.
Through four days of camping, the filter worked very well. The bottle looks like an oversized sports bottle, with a 3-stage filter inside. The Exstream has a prefilter, a cyst filter, and a "Virustat" cartridge. This combination removes virtually all the harmful organisms, including bacteria and viruses.
To use the bottle, you fill the bottle to fill line, insert the cartridge into the lid, and screw the lid onto the bottle. A squeezing motion on the bottle pushes the water through the filters and out of the drinking spout.
I used this as my primary filter for the four day campout. I have to admit that after prolonged use, it does get a little tiresome squeezing the bottle. Yet, it never failed to do a great job. We primarily used a small creek that was fairly clear, but had a lot of sediment. I would just carefully draw water from the top and filter away.
Lightweight (7 oz) and easy to carry. You don't have to stop and pump water while hiking, since you can simply fill the bottle and then drink whenever you wish while you walk. It does a great job in filtering water with a good taste. My Exstream cost about $45 from a local outdoors store.
Filter speed is not quite as fast as using the MSR MiniWater (my friend could fill his Nalgene bottle faster, even when he had to stop and clean his filter). Squeezing a large volume of water can get tiresome, but this can be offset somewhat by filtering half of the Extreme then refilling. (The bottle is easier to squeeze when full.) There is no way to clean the filter if it becomes clogged, but I had no problems with the filter. The Virustat cartridge is rated at 26 gallons before it must be replaced.
I really like this filter. If you want something lightweight, it is a good choice. For larger groups, you may need to consider something like the MSR filter. I like the idea of having water available anytime without having to stop and pump."