Maybe it's been a while since people have discussed flexible filaments, or I didn't look hard enough. I was disappointed I couldn't get Ninjaflex to work when I got my R2 after having it work on my R1+ modified with a guide tube in the extruder. I kept meaning to go back and work at it some more, but ..... I just bought a $5 sample (50 grams) of gray TPU Sainsmart flexible filament from Amazon and it works GREAT in the R2! I'm so happy! I love printing things with flexible filament. It's 95A durometer which is flexible but not super soft. Somewhat like a soft (rubbery?) pencil eraser or an iPhone case. I'm using no retraction, an extrusion multiplier of 1.10, 215C and 50C on the Flexible Removable Buildtak plate (if you haven't bought that yet, do it NOW, it's awesome), 900 mm/min, cooling after first layer, and whatever standard other settings came up from Simplify3D when I auto-configured a new material to start my TPU setup. I could answer questions if I didn't put enough info here. It sticks well for printing but pulls right off afterward when cool. It might print faster, but I haven't tried anything other than my starting values yet.