Traditional pen plotters usually don't take ordinary pens and run on HPGL instead of G-Code (and I for one find HPGL a lot easier to generate and debug). The AxiDraw sure looks awesome, but you can probably pick up four working used HP 7475A pen plotters or similar on eBay for the AxiDraw's US$475 price.
Don't get me wrong, EMSL makes really awesome stuff. You just don't need to go for that kind of premium cutting-edge engineering in order to play with a pen plotter.
Maybe even more fun is to build one yourself. They are a great introduction to CNC hardware without some of the added mechanical complexity involved with something like a router or mill. The parts are fairly cheap because they don't have to be super rigid and the steppers don't need a lot of torque, plus there is quality open source software to run them on cheap microcontrollers (grbl on Arduino + a cheap motor shield from China). There's tons of videos on YouTube of how to build one from scrap DVD drives for instance, e.g. https://www.youtube.com/watch?v=0PmR_1dx77s
There's an introduction to HPGL at http://www.tobiastoft.com/posts/an-intro-to-pen-plotters. It's really easy, and you can get started a lot more cheaply than with AxiDraw.
Don't get me wrong, EMSL makes really awesome stuff. You just don't need to go for that kind of premium cutting-edge engineering in order to play with a pen plotter.