Summary: Dissapointed
Comment: I am a Canon fanatic and own many lenses and this lens is the one that does not give me that happy glowing feeling. I bought it primarily so when I went to sporting evens I could get closer into the action - I tend to buy seats close rather than sitting at the back of the stands and I am frustrated after a year of ownership.
At 300mm the lens is blurry, it seems to produce inconsistent color when I use bursts (1-2 pictures out of ten come up dark.....?) and it can be slow to focus. I thought I would be able to use this for nature photography however without the predictability of baseball I am not sure I'd be able to snap a birds that might land quickly and dart away again.
Its just not up to the quality of my other Canon lenses. I also think it is shameful that it does not come with a hood considering the price - you can tack on another $40 for that. When all is said and done you may find something similar from Sigma or one of the other producers that is cheaper and of similar quality. I personally wish I had gone that route for once.
It is not bad ... its just not great. I'm looking for a replacement.