Let me start by saying I really love this lens for what it is. I also have the Canon 24-70 2.8 II and I see little difference between the 2 except I wish the canon had Image stabilization oh yeah and the huge price difference. Other than that, this lens is super sharp, very fast and very steady. It seems built well.
I love how easy it is to obtain focus in manual mode. the adjustment grips move nicely but they are backwards from what I am used to. The focus is in the back. Not a problem though, in just a few minutes I was used to it.
I have noticed that the corners vignette at f2.8 but that is a easy fix in lightroom. A funny quark is that lightroom reads this lens as a Sigma 24-70 so you actually have to go in and manually put it in for lens correction.
I love the sharpness of this lens and I really do not see enough difference in it and my canon to have justified the extra money on the Canon. I wish I would have found this before.
Where this lens excels or steps ahead of the canon is handheld low light. If your like me, this will be most of your shooting conditions, no flash hand held lower light. The IS or as they call it VC handles this wonderfully keeping blur down and providing you with a lot more usable pictures.
Where it suffers. The reason foe the 4 star there is a lot of lens flare when shooting towards the sun. Really bad distinct lens flare that you do not immediately notice. I did not notice until I was editing and it was too late to get better shots. I am not talking about beautiful artistic lens flare either, I'm talking about a washed out overexposed circle in your picture. I will try to upload a picture with it to show you what I am talking about.
All things considered, I highly recommend this lens, I seriously doubt most of my other lenses will have much time mounted to my camera from this day forward. If you are on the fence, go ahead and get it, you will be happy you did