So for the past week I was in the Virgin Islands on travel. Aside from a handful of landscapes (16-35 f/4) and at the wedding we attended (50L), I used the 100-400 + 1.4III a majority of the time. Every morning I went out and photographed birds, took the lens on a hike in the National Park on St. Johns (HIGHLY recommended if you visit), and it went to the beach with me a few times. The lens easily handled the birds and wildlife, but also some product shots for the wife's apparel business and portraits of said wife. That being said, it came with a price: the weight and bulk. While it's not unmanageable, it gets heavy. If it weren't for the fact I wanted to shoot birds, I wouldn't have brought it. I'm glad I endured the heft though because it really delivered some awesome results. In the future, I could see traveling with just the 100-400 and 16-35 f/4.
I used the ThinkTank Turnstyle 20 for general walk around. Was able to carry the 100-400, 1.4x, and 5D4 w/ 16-35f4 attached. That was about the limit for that bag. ThinkTank Airport Essentials was my carry-on/carry-all: AWESOME bag for travel.
Ok so I've been following this thread for quite some time now...trying to convince myself that I don't "need" another lens. But I broke down and bought one last week. Now I can understand why so many of you seem to love this lens!
A quick couple test shots. As for BIF's, it is definitely a LOT easier than using the 500mm prime. Not sure if the near-instantaneous focus acquisition is due to the 7DII and/or this lens (previously used the 50D for such images). Looking forward to trying it out on the 5DIV.
Welcome to the club! Great lens, right? You'll find here shortly that it's the only lens you use anymore 
Canon 100-400mm Mk II comparison; native 400mm fl vs 560mm with Canon Extender EF 1.4 II
As you demonstrate, the 100-400II + 1.4x are like peanut butter and jelly.



