The Tamron 17-35 2.8-4 is an absolute bargain, light, cheap, perfectly usable.
Not L lens quality but certainly holds it's own against the 17-40L (which isn't Canons finest L lens)
It seems fine enough if you don't want much lens flare in your landscape shots!

Have a look here:
http://www.kenrockwell.com/canon/lenses/17-40mm-performance.htm#compare
There's a lot more info there in that review if you follow the navigation tabs, however, it seems to compare the 17-40 L with the
16-35 f2.8 L (Mk 1?), rather than the f4 IS L version. However, it does demonstrate that the 17-40 isn't too bad at all with lens flare and ghosts.
As you'll see from that review, with the 17-40 L the edge softness decreases as the lens is stopped down, and by f8 and f11 it doesn't seem bad at all, and fairly comparable with other top quality, ultra-wide zoom lenses. You don't get IS, but on an ultra-wide lens this probably matters less than on a tele or super-tele, as camera shake movement isn't magnified to the same extent. If you use it on a tripod for landscape work you'd have the IS switched off anyway, and also how often would you be shooting at f4 or f5.6?
I don't have the need for an ultra-wide lens very often, but was aware I hadn't got this range covered, so I opted for a used 17-40 f4 L as I found one at a decent price for a mint, boxed, made in 2015 (according to the serial number) lens from a well-established camera shop with a good (hopefully!) guarantee. I think the 17-40 L represents good value for money
second hand, if you can find a
mint, recent one (they've been made for quite a few years now), from a reputable, long established dealer or shop with a good guarantee, for less than £399 (prices seem to be rising not falling at the moment - presumably due to the current £ exchange rate).
However, I'd agree with the other posts here; if money was no object (or I used an ultra-wide lens a lot, or shot hand-held in low light, or needed optimal image quality for professional work), then I'd definitely go for the 16-35 f4 IS L, which seems to be well thought of.
So before parting with your money, check out some reviews, do some pixel peeping (for colour and contrast, as well as sharpness!), and have a look on Flickr for some 'real world' type landscape shots (preferably full resolution type images, rather than ones that have been reduced in size or otherwise significantly modified) and see what you think about the difference between any options you're considering. Is a gain in image quality worth the price difference to
you? Would any saving be worth it to
you, or would you get that niggling feeling and wish you'd paid a couple of hundred £ more for the best quality you could reasonably get? The choice is yours! Hope this is useful.
PS Good luck choosing, I hope you get a good version of the one you buy... and don't forget to tell us what you finally chose, and why!
