Excerpt: I got one of these in a recently purchased 600f/4L IS. The loss of light in a f/4 lens is significant in my opinion. I had to use much higher than I'd like ISO for my taste in sunny conditions to get fast shutter speeds. Resulting in softer detail on birds and such. Not what I expected. I also find the over saturation of blues to be less realistic than I want to deal with. Focus with a 1.4x extender is slower than it should be as well due to the loss of light.
Excerpt: I just bought the New Canon 200-400 1.4X Zoom lens and during a recent shoot I noticed a lot of glare off the windows and hoods of the race cars I was shooting. (Not due to the lens) I purchased the Drop-In Circular Polarizer and it worked great! You simply remove the factory carrier with the clear filter installed and drop in the polarizer. There is a small wheel on the top which is easy to locate and rotate the filter reducing the amount of reflected glare.
Excerpt: This drop in filter was a great way to go to make it easier to keep the filter clean and protected. I particularly like being able to adjust it with the thumb screw rather than reaching to the front of the lens.