Thanks to adafruit, you can now print using NinjaFlex on a Printrbot. They have created a 3D printable add-on that when put on the extruder prevents the Ninjaflex filament from buckling (a major issue with printrbot printers).
The design can be downloaded here: http://www.thingiverse.com/thing:337267
http://www.youtube.com/watch?v=2EAOJjQMxFE
Has anyone tried this yet?