While the idea of being able to draw 3D designs sounds attractive, and while it may seem 3D extruder pens might be the missing alternative. I'm here to say that they draw more in like
2.5D, in that yes if done properly they can draw things according to the user's hands in free motion. But, achieving true free-form 3D is beyond us right now. It's also extremely painstaking to try to draw layers with your hands. The benefits of a 3D pen is more for artists trying to bring flat works into "pop-up" flat works. The structures "printed" from these pens are not structurally sound, tend to lack precision (as the extrusion is both controlled by your hand and your artistic ability), and currently their material of print is the low quality materials such as ABS, which has issues in regards to temperature of print environment. (Depending on your surrounding environment prints can curl up or droop based on the the temperature.) This was a problem the Replicator and Replicator 2's had to actively deal with and to an extent are still dealing with.
The basic principle is also similar to a high resolution hot-glue gun so don't get your hopes up of free-wire entanglements as well as having to continually feed plastic rods through the back of these pens. The size of the swissPen, 3Doodler, and 3DYaya are also not to be trifled with. The new lix pen being the only artist friendly form factor so far. I'd say if you have a machine don't give it up. These pens are not going to replace their usefulness in any way. Not unless you're a robot and can draw perfectly layer material together without any shaking from the hands. However, it is definitely an alternative to mocking-up concept ideas on paper. Being able to see Z-Axis prints can help in some aspects of the design process.
For more information on 3D Pens feel free to check out the websites of each of the makers listed above.
Some common cures for warping as provided by the
makerbot blogs
