Wow. Just Wow. The greatest eyeliner I've ever used. The eyeliner comes in a pen, and the liquid flows smoothly through the pen's fabulous brush. The brush draws a beautiful line, the thickness of which can be easily varied. I've found this pen to be almost goof proof, but if the line happens to feather a little, I blend it with a cotton bud before the line dries. (Note that once the line dries, it does not move. You need a decent eye make up remover to take this fabulous stuff off.) In terms of cost, I've been using the same pen for four months, and the liquid is just as perfect as the day I started. If you've used gel eyeliner, you know it almost always dries out long before you finish the pot.
Incidentally, the black is very opaque. I think most people will find a pencil works better for lining their lower lids.
If you're looking for black eyeliner, you will not find anything better than this. I know, because I've tried everything out there.