[Rover]

the corners at17mm of the 17-40 are terrible.

At 17mm, yes - that's why that lens is so cheap - but a lot better at the longer focal lengths, even at 20mm (somewhat). The 17-40 seems to have very little of the wacky field curvature issues of the 20/2.8 (that really seem to drive the resolution into the ground unless you carefully manage the subject distance) and similar vignetting at comparable apertures (way less than what the 20 has wide open). Plus you get the versatility of a zoom, weather sealing and better build quality for a sum that isn't radically greater. The used prices are usually good because the 17-40 is a "starter" zoom and a lot of people were getting it as such but offloading it later in favour of the 16-35/2.8 and whatnot.

 

Well, *personally* I wouldn't have bothered with the 17-40 either - I was scared by the very Photozone review that indicated miserable corner sharpness at 17mm, even though I was never a FF user. That, and I wanted more speed at that time, so I went with the 17-35/2.8 L and then with the 16-35/2.8 L II. Now, of course, I couldn't care less about the 2.8 aperture (well, mostly) so I chose the 16-35/4 L IS. I can sing more praises to it... again and again.