I recently purchased the Sigma 20-40mm f/2.8 EX lens because it was similar to the Canon 16-35mm f/2.8 L except less than a third of the price! Because I figured I wouldn't be able to afford the 16-35 L, I bought this one and used it happily for a few months. This lens is very sharp, autofocus is accurate and fairly fast (could be faster) and it's built incredibly well. The 20mm wide and 40mm long are great focal lengths!
Then, out of nowhere, I found a GREAT deal on the 16-35, and so I bought it. As a result, I don't use the Sigma 20-40 much anymore, and am selling it as a result. But to be honest, it makes me sad to sell it because it truly is an amazing lens! If you can't afford a $1000-$1500 lens with this focal length and aperture, get THIS ONE. I don't believe Sigma makes this lens anymore, but you can find it here on Amazon and other places if you look hard enough. I wanted to keep it as a backup lens, but... I just figure someone else can have a good time with it! I will miss you 20-40mm :(