I just bought a MSR Hyperflow. Used it on one trip so far, no problems. My understanding is that the negative reviews on REI.com are all from before MSR fixed various problems with it before 2010. Took 'em mighty long to fix something so important!
So far I'm feeling unimpressed with the bottle attachment system. It works great if you have ONE Nalgene wide mouth bottle, as the filter output attachment screws right onto it, but if you have to screw the special cap onto every Nalgene bottle (or Cantene, as I use) you have, it gets annoying. If you have a Camelbak, instructions are to pull off the bite valve and pump directly into the hose -- a great way to spill water or damage your bite valve -- I elected to get a separate hose so I could use my quick connect adapter instead. If you have a Platypus system... you're in luck! The Hyperflow plugs right into those without any adapters! Unfortunately if you want to backflush with a Platypus, you're going to leak water everywhere because of the design of the Hyperflow's nozzle, making using a Nalgene or Camelbak the preferred method to clean the Hyperflow.
With the Katadyn Hiker I was able to plug the output hose right into the bottle adapter, which plugs into any wide mouth or narrow mouth Nalgene -- you don't have to screw it in -- and pump away. With the Camelbak, I just connect the quick connect filter adapter and click it into my Camelbak. I am probably going to borrow the bottle adapter from the Katadyn system to use with my MSR Hyperflow.
On the other hand, the Hyperflow seems less fragile than the Hiker (on a previous trip, I broke the outlet nozzle off, which Katadyn replaced promptly), and does seem to pump pretty quick. Best part for me though is that it takes up less space in my pack and weighs half as much. I'm undecided on which way I'll go moving forward: the Hiker is tried and true, runs for dozens of gallons without needing any kind of maintenance, and pumps nearly as fast, but the Hyperflow is lighter, seemingly more durable, and smaller.