It's a very long time a have it now, and still unhappy this pump system
What was thinking the engineers which develop this lens seriously the most un-ergonomic IMO (total opposite to ergonomic) lens i ever used !
Anyway, i use it at 99% at 400mm.
Image quality is very good, sharp, plenty of contrast, the AF can be quite slow some time, but very accurate and spot on
There is an urban legend about getting some dust inside, personally i don't have dust inside after ... a decade may be.
I just regret to not have bough the prime 400mm f/5.6 instead, because whatever you are thinking > you will be most of the time at 400mm, and even often it's too short.