I have the 70-200 IS F4 L and I'm quite pleased with it. It's compact, light weight, and has excellent IQ. I also debated purchasing the 100-400 L but declined for two reasons. I did not like the push-pull zoom and a lot of user reviews comment on softness at the long end wide open. What I did instead was to purchase the 300 F4 L which had rave reviews for sharpness and image qualities. This is what I principally use for wildlife and bird photography. The focus is also quite fast. I also purchased the 1.4X extender over the 2.0X extender because most reviews say there is no image quality loss. It works with both the 70-200 IS F4 and 300 F4 L. For wildlife stuff, the 1.4 is mounted on the 300 F4 L most of the time.