As UWA lenses are used for my main photography I had the most known uwa's in past :16-35L I & II, 17-40L, Tokina 16-28 2.8, Tokina 17 3.5
If I would't have this 16-35 f4L IS, I would have stayed with Tokina.
If you want to upgrade your 17-40L I would advise :
1. Tokina 17 3.5 ATX-Pro
If you enjoy a light compact sharp UWA prime (they are seldom, this one is a gem!), very sharp, no flare! filters! and if you don't mind buying on Ebay - not produced anymore.
This one is even cheaper than your 17-40L, I didn't mind having no zoom for the small range and you can still crop.
2. Tokina 16-28 2.8 ATX-Pro
If you don't mind no filters, heavy, flare, built in lens hood, corner to corner sharpness.
IQ wise it's perfect, if you shoot with bright light falling in you need to learn to avoid the flare.
If you want to stay Canon, the 17-40 is a great price, I even prefer it over the 16-35 I & II, I just didn't like the corner unsharpness.
I just took out of hand shots with IS at all f-stops and all corners are sharp, unbelievable!
JohnPh wrote in post #17171036
Have you used the 17-40 before you bought this? Interested in real world comparisons