Update 6/18/2013: Here are a few thoughts after shooting the lens in my backyard.
1. It is more hand holdable than originally thought. While it weighs more than the 500, it is more compact and seems to balance well with the 1dx. Now, I don't mean hand holding for long periods of time, but for tracking a bird in flight throughout the scene, with a break in between, it should work well.
2. The focal length range is very useful. As I pan around our pond and imagine the migrating birds that come here in the winter, this lens will be a one stop shop.
3. The built in teleconverter is a brilliant idea and works flawlessly. It is almost magical to see the image come closer at the flick of the lever. I can't wait to get back to Africa and laugh at the dust. The lens will never have to come off the body.
I've been waiting for Canon to release a 200-400 zoom lens ever since I was able to try the Nikon version several years ago. So, for several years. Then, in early 2011, Canon announced the development of the lens, and it had a twist: an innovative feature of a built-in 1.4x teleconverter that could be activated by moving a lever. For a wildlife shooter, this meant that I could cover the range of 200mm to 560mm without having to change a lens or add a teleconverter. It seemed like a particularly well suited lens for a lot of the stuff I photograph, bears, African wildlife, etc.
So, after a more than two year wait, it came. I thought I would share the experience of unboxing it and created this video while I opened the box.
I'm sure I will have more posts about this lens. If it realizes the quality indicated by early reviews it will probably become my most used lens.
It will also mean that I can sell my 300 and 500mm lenses. This will make the pack smaller and lighter, which is not insignificant as I get older.