I use sharpies extensively in my work. Especially in my paintings. I used to use them to do my line drawings and then would work to cover those lines with paint. I find it difficult as Sharpies tend to bleed back through the acrylic paint. So, I decided to start incorporating that effect into my pieces. it works nicely.
I also use Sharpies in my illustration work. Some pieces require a heavy non finished line so I use Sharpies for that. As well as for filling larger black spaces on other more detailed work.
My pencil box/painting table is never, ever without a Sharpie.