I've been running one since I put the new 'plant together. Took a bit of fiddling with the engine running in and tightening up. The most visible benefit is is no oil film seeping around seals (valve cover grommets being the most visible). I run a fairly healthy cam (235*/.55") and fairly high compression for pump gas/iron heads.

For my own unscientific testing, sitting at idle with a regular PCV valve, I'd get regular puffs of vapor out the breather (running an open K&N). With the Wagner valve dialed, no visible vapor from the breather at idle. I haven't figured out a way to check the cruise flow for some positive benefit, but no oil film seems to be all that is observable.

Dyno and track time will show it to work as advertised, I'm sure, but clean engine is the most notable benefit to the average street car. in my mind.