I purchased the Platypus GravityWorks 4.0 L Filter System about a year ago at the recommendation of a trusted backcountry buddy. I have since used the system on several trips with no complaints. I'll admit a $120 seems like a huge investment, but definitely worth the cost. Super easy to use (just fill, hang, and wait). Super durable, mine's been used and taken in and out of my pack at least a hundred times with no signs of wear. Very convenient, with a multitude of adapters to fit everyone's filling needs.
I did notice a lot of partially negative reviews in regard to filter longevity, or lack thereof, something one can avoid by following manufacturer recommendations and back flushing every use.
I am glad I purchased this filtration system and would highly recommend it to anyone!