Peter Burke wrote a review of MSR HyperFlow Microfilter on February 2, 2009
I've used water filters in the Sierras since the late 1980s and was hoping that this highly rated unit would make the act of pumping less of a chore. When new, the thing really flows, but it only takes a few days and the filter will clog, even though I was using it in rather pristine clear water with prefilter. After 18 days in the mountains and only maybe 50% of the water we needed the flow was down to a trickle even with a lot of pressure applied on the pump. The worst part - the alleged back-flush option is a virtyual no-go. I sure could not figure out how to do it on and off the trail, and I am somewhat of an engineer. The thing simply doesn't budge when put together in backflush orientation. This year I plan on doing what most ultralight hikers do: bring a small bottle of bleach for water that may be suspect. Lighter, failproof, and considerably less aggravating than a pump that starts out fast but turns into a clogged pain after just days of use.