At the risk of incurring the wrath of 17-40 fans on here, I notice Justin's review does not feature an MTF50 test. So I find it lacking.
Have a look here:http://www.photozone.de/canon_eos_ff/427-canon_1740_4_5d?start=1
Keep in mind these are my own opinions, most of you will probably differ. However, you won't be convincing me that I'm wrong from my point of view, because I have done some research, and I'm not wrong. Hopefully at least some of you can relate to some of what I offer, though.
My own photographic interest is not limited to only the focal lengths wider than 20mm, or even 24mm...and if they were...see my last paragraph. I'm also not interested in only being able to do daytime "landscape photography", although that would still be one of the primary uses for me.
It looks to me like the Canon 17-40L is less sharp and has higher CA, higher vignetting, and more barrel distortion, than say the Tokina 16-28 f/2.8...besides being a far slower aperture lens to begin with. Of course photozone's test of the Tokina didn't seem all that promising either, because they tried several copies and all were decentered. The Tokina also seems to have other problems, such as flare and the huge bulbous front element.
I'm currently in the market for a wide angle lens. Having looked at several tests and sample pics, from most of these lenses, it seems to me that the 14mm Samyang/Rokinon/Bower lens is almost on par with the Zeiss 15mm f/2.8 and Nikon 14-24. And it only costs $300. Google search tests of this lens, under Samyang...
However, I don't think I can make good enough use out of a 14mm prime lens, and would prefer a zoom. Also if there is any architecture then you throw away a good bit of that 14mm, along with sensor resolution, to correct the mustache distortion...then of course there is the temptation to shift in post, in which case you wind up with less resolution than an iPhone...and then if you also want to correct some of the HUGE rectilinear projection distortion, then there goes the rest of the resolution.
Again the Tokina 16-28 has some flare problems, besides apparently being hit or miss with getting a sample that is properly centered. I am however still heavily considering it.
So the Canon 17-40 does not look all that impressive to me. It's decently priced, but is very similar in price to the faster aperture Tokina. Ideally I would like to do wide field Milky Way shots, so the 17-40 is just too compromised for that. Very high vignetting, very soft in the borders until it goes past about 24 to 28mm focal length. It's probably usable for tripod-only photography from f/9 to f/11, but that does not endear a lens to me very much. Most lenses are able to do this at a bare minimum, even decentered ones.
The Sigma 35mm f/1.4 is clearly the best wide-angle lens available of them all by a wide margin, but just isn't all that wide...and isn't a zoom. If I wind up doing low light stitched panoramas, my Voigtlander 58mm f/1.4 seems even better suited, and is not much softer than this mighty Sigma.
The Tamron 24-70 f/2.8 VC is clearly the winner overall in my opinion, but may not hold its value very well on the used market, because most everyone already sees it as the "bargain lens" (because the Canon 24-70 ii costs nearly double)...so they just buy a new one, rather than a used one for $200 to $300 less. So the hit for reselling will be more like $350, at least based on amazon marketplace (the ones listed higher tend to stay on there forever. I prefer to buy a lens like this and use it a while, then sell it...so in this case it would be cheaper to rent...but then the time is too limited. This is probably just me, though.) Also the Tamron's background bokeh is supposedly not all that smooth, but then it's not really much worse than the Canon 24-70 ii there.
The Sigma 24mm f/1.8 is clearly a bit better than the much maligned Sigma 20mm f/1.8...it costs less than the 20mm as well, besides costing less than the fully manual Rokinon 24mm f/1.4. However the Rokinon 24mm probably has a slight optical edge. Certainly the corners on the Sigma 24mm don't get usable until f/9 to f/11. From the product pics, it appears this Sigma on Canon, has a new focus ring, where the Nikon versions do not. All variations of this lens are now out of production. Probably a big negative.
The Canon 16-35 ii can work but is too costly for its optical performance, in my opinion.
So if cost is no object, just buy the Nikon 14-24 (w/adapter) or the Zeiss 15mm or 21mm. If cost is a concern, then I still don't know just yet. The Canon 17-40 just seems too compromised for me, though.