Printing with NinjaFlex is tricky. The filament path needs to be constrained so it doesn't buckle when being driven into the extruder.

I found that this extruder upgrade is necessary for my Makerbot Replicator 2 - but easily printable in PLA. The only changes I made to the standard Makerbot print settings were to slow the printing speed to 30mm/sec (230C print temp).

I also have a Printrbot Simple Metal - Ninjaflex not printing as well on this yet but will post details if I have any success. The extruder is jamming when loading fillament. I need to play with the extruder temps and modify the upgrade to the hot end I am using.