I have a set of magnetic lenses. I didn't try out clip-on holder because camera lens is quite far from top of the phone and I didn't want holder to block "half" of the screen
Magnetic connection is quite strong and so is the adhesive of the ring after it has some time to set.
If you want to use the lense also on front camera, clip-on might be only possibility. FFC does not have enough room for the ring. Mine has inner diameter very close to diameter of the camera lens (
see this thread, some images also on page 1).