I harvest yeast for direct pitching, so I employ 3 lines of filtration - I use a coarse nylon mesh bag inside another coarse nylon mesh bag for the hops in the boil (I suspend the bags with a clamp and they freely float in the boil; this filters probably 40-50% with no noticeable effect on IBU's utilized - though I cannot say that I have my beer tested). Then I have a standard SS screen in the bottom of my keg with a dip tube running through the center of it (I'd guess that filters another 20-25%). Finally, I use a double SS meshed colander sitting inside another SS colander to catch any break material and hops that might get through. I rarely have any noticeable vegetal materials in my fermenter and my yeast harvest is nice and clean for the next batch. I typically brew 10 gallon lager batches and it seems to make a difference with minimal extra cleaning. I remove the hop bags as the wort cools, so the bags are done being cleaned before the wort is transferred. The rest cleans up easily, as everything is removable.
The Hop Stopper looks pretty effective, though, so I may give it a try and dispense with the bags and boil screen in the kettle, as it would serve that function rather nicely, or so it seems.