I guess I'm in the minority in my opinion of this lens but I was quite disappointed with it in practise. Sharp enough out to 400mm if stopped down, but beyond that it didn't actually resolve more detail than you'd get by cropping that 400mm FOV. Certainly a borrowed 400mm f/5.6 out resolved it at 600mm considerably.
AF with the 70D I had at the time was ok at 400mm and below (albeit a bit slow), and rather hit-or-miss (mostly miss) beyond that once it hit the f/6.3 zone. But, even manually or live view focused images at 500 or 600mm were just too soft for my eyes.
If I could live with it being a 400mm lens I'd be happy enough - but it's rather large and heavy for a 400mm lens.
Depending on your subject matter some of these things may or may not matter. But value for money wise while it's obviously the most cost effective way to 600mm there are certainly some limitations beyond 400mm and it's that part of the focal range that was most important to me and is what impacts on my review ratings the most heavily.