I have 2 different machines that have knee lifts and I can't use either one. My upstairs machine sits on a desk with drawers on both sides, leaving no space for the knee lift. Also, to have my extension table legs on the desk, I have to push the machine back to where the horizonal part of the knee lift arm is too short. My downstairs machine is strictly for quilting and is surrounded by 4"-deep styrofoam, so no knee lift there.
I don't know about my downstairs machine/knee lift, but the upstairs one can be taken out of the machine very easily; re-inserting it is a major chore. It may be a flaw on this particular machine/knee lift or may be common to all the Janome machines. Either way, not a good thing!