It doesn't convert properly, unusable totaly with vray. Add for example layer in diffuse channel than add texture + color overlay, you'll end up with black diffuse in corona, and so on...
Same with C4D materials and layer shaders.
It is not perfect, yet it helps!
I usually do an automatic conversion, copy all original C4D/Vray materials over, place them next to their converted counterparts and start a manual adaptation. It saves time, because I already have the full number of Corona material, most channels needed are already active (and some even work already!) and I can copy over from the original material what I need to the converted Corona material.
It is not something I love doing, but creating all materials from scratch is far more tedious.