This lens is a WA, bordering on UW, and it is the fastest 24 mm around. IOW, it is bound to have slightly softer corners than the centre is. It also used to be the world class leader in its class (+/- 25 mm lenses for FF and APS-C), and I only say used to be, because now we have the 24L II, which is slightly better again.
The question really is, what do you find acceptable, for real life use, and will you notice these slightly softer corners? If you shoot buildings head on, I guess you will if you don't stop down to F/4 or F/5.6. But that's fine IMO. If you use the lens at or close to its maximum aperture, you will likely use it for low light shooting. Will you notice any slightly softer corners in that case? No, not IME. You could use this lens for street photography. Would you notice the slightly softer corners in that case? No, you won't. Landscapes? No you won't, because you stop down. WA effect shots? Neither. Etc.
This lens is better than any of the zooms covering 24 mm available (16-35, 16-35 II, 17-40, 20-35, 24-70, 24-105, 17-55, 18-55 - all incarnations, 17-85) at 24 mm, not only for sharpness, but also for speed: it is 2 stops faster, at least, than anything else in Canon land.
And apart from that, it renders very, very beautifully, great contrast, colour and bokeh (especially for a WA!) straight out of the box, so to speak. It is in the same league as the 50L and 85L when it comes to that.
Do I like it? Yes, you betcha. And I know I am not the only one. People who have one, or the Mk II, generally don't let this lens go.
Kind regards, Wim
Wim. This post is what finally sold me on purchasing a 24L. Good no nonsense look at the 24L.