Hard to put opinion on this.
Have owned it for 3 months now.
The first thing I had to do was hand it in when AF was completely off.
It is naturally sharp, fast AF, solid and so on. But for the price one expects it.
Along with my 5D MkII, I am not completely satisfied. AF on the camera is not right it should be, which is incredibly important in this lens. So you have a 1-series = najs, but otherwise you have to take it a bit more cautious ...
Did cleanly with 1.4 Extender. Easy deterioration, but there is no problem to live with.
With 2-Extender is a different show. Obvious deterioration, but still a good stopgap. One must, after all, 600/5.6 with IS and right ok image quality. Aperture man down 1-2 steps, well then it will be fine. But that's not so common in practice.