I think BT cable is a must-have if you are going to tune. I had it for 2 years with my JB4 tuned 135i and I always used it before any warranty work (HPFP issues) and regular maintenance. It NEVER let me down.
I downloaded a new file and now it works very well with my F30 and showed the error codes associated with an electric fan malfunction that the dealer was denying!
Great investment if you are getting the JB.
Gone but not forgotten: 2008 BMW135i Vert
Gone but not forgotten: 2011 MB C63 AMG
Gone but not forgotten: 2011 BMW z4 3.5is
Added: 2015 Z4 3.5i