I think that the Canon lens is the best optically that is currently available. But how many people us a lens this wide and shoot wide open? Usually the main use of such a wide lens is to get close and bend perspectives into creative shapes. So stopping down is usually required. The cost and size is a stopped for me, I would use a lens like this so little it would be hard for me to justify the bag space and ticket price. I personally, would be far better served with a good copy of the Sigma 12-24mm mkII. Only you can make that choice.
The Sigma is nearly as wide, it's a lot cheaper, lighter and smaller. If you are stopping down, I doubt there is much difference in image quality unless one is being very very pikki.
Canon 5DII, Sigma 12-24mm. 12mm @ f8. The lake in the foreground is just a small puddle about 24" across. Get up close with a tripod at rack to 12mm and it looks like a small lake.