I discovered watercolor pencils sometime in high school. I started using them exclusively along with a crowquill pen and Koor-I-Nor ink.
What I love about them is how versatile they are and how beautiful the color is. Being in a pencil form, you Koor can use them like a colored pencil, then brush a little water over them for some cool effects. Or you can scribble on a separate piece of paper mixing the colors you want in the proportions you want them, then use a brush to color with them like watercolors. Being watercolors, they are translucent and capture light beautifully, reflecting absolutely gorgeous colors. If you spray fix them, then they brighten (and some colors darken) like pastels for even more effects.
My favorite brand of watercolor pencils is Crayola (and they are by far the cheapest). Crayola offers an 8-pack with a brush, or a twelve pack with no brush. The boxes open up like Prismacolor boxes with a slit in the middle so you can set them up at an angle at your desk. My second favorite are Design (now discontinued). Prismacolor comes in third. I've tried other brands, but they've been very disappointing. My favorite paper to use them on is bristol board, though sketch books work amazingly well. Sometimes I use waterproof ink, and sometimes I use water-soluble ink to give my drawings darker shadows. Again, watercolor pencils are so versatile, you can use them with anything!
This drawing is one of a series of three I did in college. I inked the drawing with a Pilot Precise V7. I was still trying to figure out my style and how I wanted to make lines. This series turned out really well, and I'm still proud of them today.