I just bought this lens for Canon mount last week and went to the zoo to take some pictures of animals. Pictures of animals that do not move that much, were sharp even at 600mm when it is taken with a tripod and cable release. Bear in mind, that I shoot only at ISO 100mm.
Without sufficient light to achieve 1/range mm speed, objects particularly those that move frequently will be blurred. The lock release was good. Once it is locked, the lens does not extend when you are carrying it on the shoulder. Since this does not overdo the pull and push of the lens, it does not makes the lens too "loose".
Surprisingly, the USD drive was fast, smooth and quiet. It was much more quiet than Sigma 170-500mm that I used to own 8 year ago. Also, I did not experience the lack of focussing as mentioned by the other reviewers when using this lens even at 600mm. It appears that the lack of focussing has been resolved.
Taking pictures using long range lens is far more complicated that the short range lens. Any slight movement by the object will render the pictures blurred. You will need to adjust the way you take pictures and have a stable tripod and release cable.
If you need to take fast objects, this is not the lens for you. You will need a prime lens with at least F4.