This is a fairly new medium to me and I am quite enjoying it because it seems to bring together the best aspects of oil paints and watercolour. Gouache is an opaque form of watercolour, and it's very versatile. I am using these sketches both as exercises and also as ways to ''feel out'' colour choices and composition before embarking on a larger version in oil.
One of the coolest parts is setting up the illustration board/paper by taping it off with masking tape or washi tape. As soon as the painting is complete, I peel off the tape and see the neat edges of the painting, surrounded by the pristine white of the board. It's like having a built-in frame.