I picked this lens up at release, it is a beauty with both extenders as seen above. I have used it with the R3, R1 and R5 II. To me it is just about perfect for action with pets too. Such a fun lens.
Bare in mind all of these are with the 1.4ext and shot through very thick 'viewing glass' at Colchester Zoo, yes there is a bit of refraction but I am still impressed with the quality of the shots.