Great travel lens
I bought this lens despite already owning the 100-400 and a 70-200 f2.8 IS. I've recently sold the 100-400. I like how short this lens is, great for travel. It's lighter than the 70-200 and the 300 is useful. I use this lens with both a 5DII and a 7D along usually if I want the added reach. It takes a bit of getting used to the reversed location of the zoom and focus rings. Not sure why Canon chose to do this, but I have gotten used to it. I find the lens to be very sharp and the IS works extremely well. My only real complaint is that the lens does not hold focus well (in MF) while zooming. Not a big deal in AF, but frustrating when focussing manually. Compared to the 100-400, I never liked the push-pull zooming of that lens and I find the 100 on the wide end not that great when working with the full frame body (a bit long sometimes). I do however miss the 400 on the long end! Still a great lens that I would buy again.