@shamanix Comparing it to Schlick on dielectrics I didn’t find the delta relevant. But on conductors, unless the dip around the 80 degrees is prominent it doesn’t do a great job at them, and for commonly authored metals it doesn’t do as well as Schlick. It doesn’t do much better on chromium which is a material Schlick is known to not reproduce well, it does better on aluminum (probably why it was shown), but on other conductors it isn’t much better and can be worse like on gold or silver.

@pzurita @shamanix What could maybe interest you though is another still fairly simple approximation that also allows to control the shape via another parameter: f0+(1-f0-shape*cos_theta)*(1-cos_theta)^5
I've added some info in a "Controlling the Shape" section in the original blog post that explains what it does and its derivation.