The 70-200 II is probably my most used lens, no doubt. In fact, with extenders it becomes even more versatile than it already is.
Alone, you have an amazing performing 70-200 f/2.8... Sharp end to end with amazing IQ.
Slap the 1.4x on there, you get some additional reach up to f/4. Still sharp is a tack wide open at f/4 at any FL.
The 2x III is amazing on the 70-200 II... Now you have a 140-400mm lens at f/5.6 which performs very, very well. Only needs minor stopping down to give very good sharpness.
So one lens (with the help of extenders) can give you a range from 70-400mm at various apertures. It also accepts tubes very well, which make it a dang good macro if you need the MFD without another lens.
Performance is just awesome with this lens though. I love it.