When I got my first full frame camera (5D) there is nothing else I was thinking then to get this lens because i've heard that it is the widest lens in the world!!
In my country, there is no returning policy, so you must be happy with your purchase after walking out the store's door, like it or not. I had to tell the salesman to take all the stocks she has in the warehouse and I tried it one by one, and I'm glad I did this. It was my third attempt from all 5 lenses that worked.
Generally, from f/4.5 - 8, i experience a light fall-off at the corners, but f/8 - upwards this symptom is gone and the image is tack sharp from corner to corner!!! The extremely wide angle is soooo exceptional and I am so loving this lens because of it.
I also own a canon 15mm fisheye and a 16-35mm II L, who are canon's state-of-the-art lenses for sharpness. But hey, you can only see the difference when magnify 100%. In other words, both the fisheye and 16-35 win over a slight margin only.
I wish it had been a prime 12mm, cos I never used it in other focal lengths....
no problem with autofocus since I usually manual focus for landscape photography...
Loving this lens for its Extremely-Ultra-Wide-Angle!!!
If this lens can accept filters, i would have rated 10!