My very first army is a Black Legion CSM army and I have done a huge number of guys using the extreme highlight method. Painting chaos black and highlighting edges with grey.
The biggest problem I have with this method is it doesn't fit with the painting style for anything else I do. If I have red, bronze, flesh tones etc I use a more realistic method of washes, highlights and shadows but the black looks a little more stylized.
I would say it depends on your desires for the look of the model, and your painting style. For me I would love to go back and redo my models using something like payne grey as the base of my black then shading/highlighting it. Live and learn I guess