7 out of 10 points and not recommendedVery good on APS-C.Soft corners on Full Frame, especially at 17mm.
I originally bought this lens and used it on my 30D in 2006, with the intention of it being my wide angle once I upgraded to full frame. Back then, the 5D was the affordable FF camera and this lens didn't do too badly on that sensor by all acounts.reviewed November 28th, 2013 (purchased for $900)
I bought the 5D2 when it first came out and it was clear that this lens just wasn't up to it. From 24-40mm it is ok, but lower than 24mm I just cannot get decent corners out of it.
It performs well on APS-C, but is now outperformed slightly by my fiance's cheap 18-55 IS kit lens in terms of resolution. It is obviously built better than the kit lenses and is a constant F4.0 aperture, but a couple of much faster primes combined with the kit lenses would be money better spent if the (minimal) extra speed is needed.
I now use a Tokina 16-28 which smashes the Canon 17-40 in every optical parameter, but is a fair bit heavier.
The 17-40 is in bad need of an update. Come on Canon, all I want is a UWA F4.0 zoom which is sharp across the frame at F8.0-11. It doesn't need to be sharp wide open!
9 out of 10 points and recommendedExcellent sharpness at 16-20 mm, very good at 24 and 28mm. Good resistance to CA. Price.Prone to flare, heavy, requires expensive Wonderpana solution for fitting filters.
I bought this lens because from the reviews it appeared to be the best wide angle zoom for Canon EF mount, irrespective of price. It easily beats the 16-35 II in all parameters, being superb in the 16-20 range when stopped down. Aparently the corners are soft wide open, but I never use this setting for landscapes which is what the primary application of this lens would be.reviewed November 28th, 2013 (purchased for $750)
Its weakness is in the corners at 28mm although it is still very good (better than any Canon UWA zoom), but at this focal length I have my Contax Carl Zeiss 28mm F2.8 if I'm being super fussy.
Being F2.8, the lens is quite heavy. As I shoot landscape, all I'd ask for is a F4.0 zoom that is sharp in the corners from F8.0-11, but such a beast does not exist. If Canon get off the a$$ and release a 17-40II that meets these requirements I would get that, but until then this lens is it.
It is prone to flare in direct sunlight, but this is a common trait amongst UWA lenses.
Don't let the filter aspect put you off; I use the Wonderpana system for CPL/UV and split ND filters and it works a treat, although the bulk it adds make storage a bit of a pain. Given that the Tokina destroys the Canon 17-40 and beats the 16-35II at less than half the price, I am more than willing to live with it.