The best thing about using different tools in art, is the different outcomes you get from each of them. Generally speaking, charcoal loosens everything up, painting is very expressive, using marker pens is very graphic and pencils give great shading and tonal work.
Recently I have become obsessed with technical pens. Purpose made for technical drawing - obviously - they give great intricacy and detail - I seem to draw in a very different way. The result is distinctive and somewhat confined in style. I'm definitely going to continue using them in my sketching, and start combining with different tools to see what new and interesting results I can muster.