I love this lens. I've gone through all variations of the 70-200, but always found myself wanting more reach as I visit zoo's a lot. I wasn't 100% satisfied with the image quality when using a 1.4x tc with a 70-200. I switched to a 100-400 and whilst the range was very good, I was very disappointed with the quality of images. They weren't as sharp or colourful as I was used to from L lenses.
I then made the decision to go for a 300mm F4 IS, and so far I'm very happy. Image quality is the most important factor for me, and this lens really delivers. Sharpness is excellent, and so is colour and bokeh. As the image quality of this lens is so good, you can crop images quite heavily without a significant reduction in quality (I have a 40D).
The only downside with this lens is the subject may be too close to you in certain situations, but that's not a fault with the lens just a question of choosing the right kit to take with you