My K-20X has been an excellent performer. Even use it (carefully!) with a 5D3 and heavy Tamzooka 150-600 combo. In those instances I find I am more worried about the tripod that the ballhead. I use a Gitzo Traveler and feel it's footprint is too small.
ONLY beef I have is that when it is extremely cold (-20 degrees C or -5 degrees F or colder) the pin used to rotate the ballhead sometimes does not take hold.
I also find it is relatively heavy compared to my ultra-lightweight Gitzo.