Got OctoPrint (OctoPi) setup and working with my Mendel90. It’s on the wifi, camera attached so saves me nipping in and out to the garage ( “workshop” 
). Works a treat!
Nightly build or stable? (Just wondering)
It says master branch (stable) at the bottom.
Go with the nightly build. Long file name support, and better integration with Cura (using CuraEngine). I’ve only found a pi binary for v14.12, though, which is not compatible with v15 profiles. http://octoprint.org/files/octopi/cura_engine_14.12/
We should really ship with Cura binaries. Just never got to that.
Master branch @Guy_Sheffer . I’d like to try out the cura engine next, but am a bit confused by the various notes, and instructions. I expect the answers are all there but just need to read it all a few times. “sudo” is not a word I use every day! 
Happy to play with with it though.