I would add, don't be tempted by the Sigma 120-400 OS, which on the surface would seem to be a good alternative. I was so sucked in when I first came out and I failed to do my proper research on Sigma. Sucked is the operative word, my lens failed (autofocus) died last March and was immediately returned under warranty. Now, three months later I've been informed that they just got the parts and I may have my lens back in a couple of more weeks. Even before failure, it was not a great performer and the OS tended to go crazy and cause the image to vibrate wildly instead of stabilize. I mentioned this in my cover letter, but it remains to be seen if this was addressed. I have since learned that a major lens rental outfit has ceased carrying it and it's 150-500 sibling due to a near 50% failure rate.
Oh, and if you're wondering, very soft at 400, needed to be stopped down at lower lengths to be acceptable, but not great at any time optically.
A very poor lens backed up by worse customer service. BEWARE
BTW- I have to add about Romy's bird above WOW! If I could be guaranteed a copy that good, I'd jump on the Canon 100-400 like a hungry hound on a porkchop! Unfortunately, I've also seen posted samples that aren't much better than my Sigma at its best, so quality control seems to be variable.