It is a great lens. I sold mine only because I so rarely use it and I find myself going to the primes more and more. Here are the “problems”:
1)Bad copies - there are definitely out there and you can understand why people are upset about it. It isn’t cheap. The internet doesn’t help either. An upset owner will make sure his/her voice is heard across multiple forums and sometimes multiple threads one same forum.. the bad press effect gets multiplied.
2)User error - It’s big, heavy lens and it lacks IS. IMO you cannot apply the 1/focal length rule on this lens. I rarely get sharp images when the shutter speed is around 1/f. Maybe I don’t have steady hands but I get best results when shooting much higher shutter speed. It is definitely a lens the photographer needs to adapt to, not the other way around. DOF can also be quite shallow when shooting wide open at the long end so you really have to be careful with your focus. Minimise movement when you focus and re-compose. (if you use center AF point only).
IMO it is the most versatile lens that produces consistently great IQ across the zoom and aperture range. I can’t say the same about the other zooms I have owned.