Slow AF (but it's a 100-400), push/pull system can be difficult to get used to, a bit heavy.
I love this lens. I took it on safari to Tanzania last year and it performed great! I thought the push/pull system would allow a lot of dust into the lens, but in those extremely dusty conditions, I could not see a difference in the photos: there is no dust in the pictures.
The lens is a little heavy. After using it for 15mins for the first time, my arm felt a little sore, but you get used to it.