I don't actually own the 120mm Macro Planar, but I have had the pleasure of using one for many different subjects. It indeed is a wonderful hunk of glass a bit over priced perhaps, but it responds very well in all categories. I personally like to have a lens that can handle more than one task. The 120mm does exactly that. I used the 120mm as a normal lens, and was very satisfied, when photographing steam trains and historical items. I could also home in on a tiny detail without having to change lenses, add tubes or other close up lense/devices. I also believe that a macro lens designed for macro work does a better all round job than a lens with extension tubes
or Proxars. I also think that the 120mm lens is what I call "tweener"
between 80mm and my 150 mm. If I owned one I am quite sure the 80mm would sit in the bag and the 120mm would be my "normal" lens.
That simply is my opinion. If I could afford one and could force myself to use the square format, I would save my coins to buy one!