Nick Vitalewrote a review of MSR Guardian Purifier System on January 25, 2018
Familiarity: I've put it through the wringer
This is a phenomenal filter. Simple to use and quick but a bit on the heavy side.
However, I broke one of them on my last long hike. It is as if the filter clogs and doesn't back-flush very well. Pump get harder and harder to use over time leading to the handle breaking off due to pressure (in my case). MSR has GREAT customer service and replaced the filter quickly. However, I could feel the new one slowing down and starting to get more difficult to pump. Would highly recommend purchasing after they improve backflushing.
For simplicity and weight, I have switched to a GravityWorks