I don't have one these and can only repeat what I've been told by Techs, both here ini person and through other sources on the web so here goes.
First, if you have little or no blowby, then there's really no reason to use this thing, since that's exactly what it's intended to fix. When I did my motor work, I discussed this with the tech doing the work and he talked me out of it. I had already installed "The Cure Vent Plus" which is a ball check valve and a couple of drilled-out holes in the center rocker box section to allow more rapid draining of oil back out of the rocker boxes and to allow air to enter through the breather holes, but not allow oil to exit. Now that's a product I could recommend. This seems to have cured my oil-in-the-air cleaner problem completely. I would also recommend an old Evo trick of stopping just short of the "full" mark on the dipstick when yo maintain the oil level.
Back to the reed valve, specifially the criticism I've heard is that the reed itself is too stiff for Evo oil pressure to open and can actually cause blowby to worsen. Newer Evo motors came with plastic gears on the valves in stock form. There are steel versions on the aftermarket that last much longer. On the other hand the plastic ones last a looooong time if the oil is kept fresh.
So, IMO, this is one of those answers to a question that nobody asked. Maybe others will have a validation of this device and swear by it, but the folks I talked to didn't see any benefit.