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
I've been doing this stuff for awhile now using the same plotter (an Axidraw). Its great fun, and I used it to make christmas presents this year (eg: https://twitter.com/josephgentle/status/947820082218000384 ). I find it really satisfying to be able to make beautiful art despite having rubbish drawing skills.
I have been following Anders Hoff [1][2]. He does some cool stuff with plotters.
I made an egg plotter back in the early 2000s based on The Art of Motion Control site [3], and then I went off to build CNC tables and do wood carvings in 3-axis, and then 4, and 5-axis machining. I am really interested in the community around making ceramic 3D printers where they mix their own clay. All creative and really cool stuff!
A friend of mine had a plotter in which he substituted an Exacto type knife blade for the pen, and plotted into vinyl (with adhesive backing) which allowed for some amazing stuff.
He did some wild murals with it (varied colors).
I can recommend the Cricut Explore Air for this. The software is janky but the machine works great for cutting out vinyl, paper, etc. Also takes pens which means I can do my own generative plotting (once I buy some pens / an adaptor.)
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.