Without using the polariser to its intended effect - that is to polarise light, the amount of light will be reduced from 1/3 to 1/2, as what Zestorer mentioned.
Used to its full effect, it will reduce light by two stops, although for my black & white films, I will compensate by 3 stops.
However ESPN might be right in that, FOR HIM, the brightness of the VF is still MANAGEABLE.
There can be a simple reason for this. The brightness of the VF depends on two major factors. The type of glass used, and the speed of the lens. If one use a f1.4 lens, using a polariser will effectively reduced the brightness of the VF to something like f4. Something quite manageable. If one has a f2.8 lens, the brightness of the viewfinder will be reduced to that of f5.6. I think still manageable.
And of course better glass used for the VF does help. People who use exchangeable screens for cameras like the Hasselblads and Rolleiflex, as well as View Cameras will understand this.