I don't think the Sigma 150-500 has been mentioned yet. In the US it runs around $500 less than the Canon 100-400. It is heavy, and not the fastest lens in the world at f5-6.3. But I love mine. I upgraded from a 55-250 with the idea that I would keep the 55-250 as a mid range zoom. After one race I had to upgrade my mid range glass, the 150-500 was so good it put my 55-250 to shame. I think it is the cheapest way to get to 500mm and 500mm is addictive.
Here are a couple examples.
At 500mm
GTA Leads The Way by
autoidiodyssey, on Flickr
At 340mm
Jaguar vs. Triumph by
autoidiodyssey, on Flickr
At 150mm
Flying Brick by
autoidiodyssey, on Flickr