I think this is a good replacement for my aging Canon EF400mm F5.6. There is nothing wrong with the 400. I want the versatility and the great reviews makes me want to replace my 400. I will be using it mostly on a Canon 7D mark ii and a Canon 1D mark iii, occasionally with a 1.4 extender. I don't want to keep 2 lenses. Is this lens worth the upgrade? Also, wonder how it will perform with the 1.4 extender. This is only a hobby for me and hopefully someday, I can take pictures as amazing as I've seen here.