If you want to see a video on Tim Holtz demoing this awesome marker, use this link: http://www.timholtz.com/vid_fillablepen.htm
Basically, it's a dual ended marker that you fill with the Adirondack Alcohol Inks (we have every single color and set sold in the store in stock now!) Use it like a Copic. The nibs come out and the ink bottle tip fits in perfectly. VOILA! Your very own marker! Customize your entire collection. Seriously folks, this is genius because Tim's alcohol inks in themselves are genius. Coupled with the dual end nibs, coupled with the handy dandy ease of refilling (Tweezers help pull out the nib), coupled with the fact that you get to customize your collection...ooops, I guess we are all coupled out! Use it with the alcohol blending solution to make watercoloring effects. Oh there is just too much to say, PLEASE check out his demo! http://www.timholtz.com/vid_fillablepen.htm
Seriously, pick some of these up, they are the best investment you will make in your paper crafting career!