I enjoy them. They make me feel like I don't have to waste $30+ on art materials that I can go without. For me, one of these suckers and a sheet of copy paper can equal 20 expensive art pencils and sketch paper.
Fun to use, but very time consuming since you have to be careful and work very lightly and in layers. I like to use colors when I use pens, but have a hard time finding pens with the correct type of yellow ink. I think ballpoints are somewhat underrated in the art world.