The Sigma Service is fast and free, which definitely makes more sense than buying therefore the dock.
Where the Dock, in my opinion, makes much more sense, is combined with the 120-300, here you can then namely also adjust the AF speed by modes (which has 120-300 a "Custom" button), and the way is to function as the image stabilizer.
So does the whole, I think, much more sense and is a quite useful extension.
I think that future objectives will support this Dock and telephoto lenses generally offers partial real value.
