I bought a small set to try out and they are handy for color washes over my ink sketches (I've made sure to get NON-water soluble ink pens.) The pencil colors are limited when it comes to subtle shades and are somewhat intensely bright compared to a little palette full of w/c tube paints, which I prefer for sketching, but entail more supplies. I only got a small set - I've seen much larger ones that probably have more colors, but mixing with what you have is faster. I often use the silver or light grey pencil to block in my shapes and design on the paper before committing my ink lines - the pencil easily erases, and dissolves into nothingness when painted over. And I don't have those annoying "search lines" in ink where I had to reduce or enlarge certain elements, which leaves my mistaken ink lines obvious.
A great advantage is that the w/c pencils give you some versatility for quick sketches on-the-go or on-the-scene, for very little bulk. I carry a little watercolor journal, the colored pencils, 2 fine line black marker pens, and a set of 3 Aqua-flow brushes, (or NIJI Waterbrushes), and THAT'S IT. It all fits into a a small, compact bag - I use a box-like bag that I found at REI. The brushes are quite inexpensive and each has a water reservoir in the handle that you can fill whenever it's convenient. Make sure you put them in a plastic bag if you carry the "loaded" brushes along after filling. I also tape a string from the pen to the brush so I don't lose the caps. Once you do, you have to fill them on site, otherwise they leak water all over your kit. The brushes are always moist, there is no rinse water required, you just squeeze the handle to get rid of any pigment you don't want on the brush. You can get them at Dick Blick as well as other art supply stores.