I recently bought a Sigma 120-400/4,5-5,6 DG APO HSM OS and let me tell you it's a big disappointment.
Unless you plan to shoot lamp posts -or any other static object- from a distance and you don't want a clear and crisp shots with details and tonal range, this lens is of little use.
The autofocus is extremely slow in gaining focus and struggles to maintain it, when I look at the shots in full resolution not even the best ones can be used in a resolution much higher than 1024 longest side.
The shots are very soft and has a poor tonal range giving me material that lacks both details and sharpness.
One of the few usable shots taken with this lens.