I like this lens (and Sigma 1.4x combination) and will not repeat the many positive comments already written. For quality, speed, zoom and price there is no other lens that meets this.
After the 1st year I did have some maddening focusing problems with this combo. I found 1/2 of the screws near the mounting rings in the 1.4x and on the lens were loose. I removed the screws, added a tiny drop of Lock-tight - problem solved.
On one occasion since my focusing started acting up and a quick clean of the electrical contacts (lens, body and 1.4x) with a swab and alcohol solved the problem.
I use the Sigma 120-300, Sigma 1.4x and Nikon D200. This combo works great for soccer or footbal and gets you right on top of the subject. 2nd D200 has a Nikon 70-200 VR.