This thing moves some serious air.  It does have a few issues, but I can put up with them for how well it works.  It seems a bit flimsy, but once mounted it is relatively solid.  It mounts to the inside frame of the window by 2 screws on each side.  The flange on each end of the fan has keyways for these screws, so it is removable.  The knob does come off if you pull on it instead of twisting it.  If it bothers you, put a drop of epoxy or super glue on it and put it back on.  It is electrically reversable, but it exhausts air much better than it draws air, and mounting it backwards would make it so you could not close the window.  The bigest problem is that it is noisy on low speed. The noise goes away on high speed, which is how we run it anyway.