From the LensRentals Blog:
So what did we learn today? Really, not a lot. The Sony 400mm f/2.8 G is exactly what we expected; a very solidly built lens that is everything construction-wise you would hope for in a big beast of a super telephoto that costs $ 12,000. It has excellent weather sealing, heavy-duty engineering between the barrel segments, a very solid chassis, and components that all appear up to the task.