Copic markers are Japanese-made markers that have been used for years by Manga and other artists in Japan. Though generally relegated to that purpose, they’re a very functional medium, an alcohol-based, refillable marker that can also be used with an airbrush. They are initially expensive to buy, but because they are refillable and so versatile, over time they prove to be less expensive than Prismacolor markers. Because they are alcohol based, they are also blendable; similar to watercolor in application, but much more convenient.
Copic markers are somewhat limited in application, and not something generally as versatile as acrylic- or oil-based media. The advantage in my case is mostly portability. For anything larger than an 8 1/2X11″ page, they wouldn’t be practical. Think of them as more for cartooning than fine art. I do a lot of caricaturing and figure drawing, for which they work well.
[Check out the COPIC Marker Flickr pool for some stunning examples of what can be done with these markers. -- cc]