User Review: Platypus GravityWorks Filter System
This is an excellent filtration system. In my mind, water purification is the toughest place to find the perfect piece of gear — chemicals take too long and muddy up the taste of delicious mountain water; filters break, clog, slow down, and take too much effort at the end of a long day, UV systems run out of batteries and the bulbs break. While this isn't perfect, it's a great piece of gear to have available.
I bought the CleanStream before leading a group of 9 people, including several new backpackers, on a long trip into the Sierra Nevada. I figured the large capacity and ease of use would be helpful with such a large group, and it came through. All you have to do is fill it up, hang it, and then ignore it, and you'll shortly have a big bag full of clean water. I've only had the CleanStream for about 6 months and it's already filtered hundreds of liters of water with no issues at all.
Downsides: As another reviewer noted, the zip closures can be surprisingly difficult to seal. It begins to run slow fairly quickly, and backflushing is a bit finicky. But these are minor problems — the benefits of such an easy-to-use system far outweigh my minor gripes.