I've been backpacking and climbing for over 20 years (I hate saying that...) and this is the best thing I've ever used to filter water. (**UPDATE 5/2012: GravityWorks is now the best thing ever!) I'd been using it off and on on shorter trips in the Cascades and was blown away - but that's easy for most filters to accomplish. So, the proof was to be found on an 8-day trip in the High Sierras and I even bought a spare filter cartridge just in case. My research had turned up some folks griping about clogging etc. but I can only imagine that they were doing something wrong or filtering REALLY silty water. True - it does slow down if you are filtering really silty water. I noticed the difference between a glacial-fed lake and some others that were lower - but we backflushed as recommended (every eight liters, or about once every other day for two people) and had no problem whatsoever.
**UPDATE: I did discover that MSR found issues with collapsed hollow fibers in some filters. Apparently they test every filter out the door and found that they were drying the tested filters at too high a temp, essentially melting some of the fibers. All warrantied and remedied a few years back.
And for those worrying that backflushing that much is too much - think again. You'll never even consider going back to another filter after realizing how much easier the Hyperflow is. We got used to backflushing right as we finished filtering and it only added about 1-2 minutes - Far less than the time saved by the Hyperflow's speed and the overall ease of pumping more than makes up for any inconvenience you thought you might have felt. We even started to routinely do the filter test at the same time because it was so easy to do in the process.
I will say that I wouldn't use it in direct glacial meltwater and I have no idea how it would do in the Canyonlands or other places with really murky water, but I'm guessing it might not be the best choice there. However, if you want to get a superlight, and really small filter for taking on alpine trips or anywhere the water is pretty clear, I can't imagine a better set-up. I even take it on day hikes and car-to-car alpine climbs now and pump as I go. It beats lugging two camelbaks around in my pack or dealing with the taste and hassle of iodine.
Oh, and I still haven't touched that spare filter cartridge.