These pens are great for drawing because there are so many nice colors available in the 18-color set. The set is not that expensive and even if you don't want to purchase that many pens, there are smaller sets available for even less money. These pens are 0.4 mm so the line is thin enough to be suitable for writing purposes. However, I tend to use these primarily for my artwork. I find that these pens are very good for shading and coloring-in. I also like underlining notes with these pens - they are a great substitute for highlighters. I also like their compact size, as they are easy to travel with and can fit in the back pocket of my jeans. I always try to carry one pen with me when I'm on-the-go at work, and this pen is perfect for that purpose. These pens are available in a larger size (check out dickblick.com), but I find them to be too long and they don't fit in most of my pencil cases. Overall, I love these pens. I really like their compact size and how they can serve as both an artistic tool and a writing tool. Also, there are so many cool colors available, even more colors than I've shown here (though they may only be available in the larger size).
A small amount of coloring in this picture was done with the Stabilo pens. As you can see, you can achieve nice, even colors using these pens.
Pic taken on a stool in lab (Yeah, we play with our pens when we have a little downtime in the lab =0)