Canon 17-40L vs. Tokina 16-28
I originally had the Canon 17-40L. I found it to be a very good lens, but I needed f/2.8 for available light shooting. So, I sold it and picked up a very nice used 16-35 2.8L (the first version). It was a good lens, pretty sharp in the center at f/2.8 and comparable to the 17-40L at f/4 and above. When I saw the reviews for the Tokina 16-28 f/2.8, I decided to sell the 16-35L and and give it a try. I would say the Tokina is overall sharper than the 16-35L and has less distortion. I do miss the faster AF of Canon's USM, and the ability to use filters, but the Tokina has served me very well and has saved me in some very tight situations where I needed a really wide lens.
So, if you really need f/2.8 and can't afford the Canon 16-35L, the Tokina is an excellent choice. If you don't need f/2.8, stick with the 17-40L.
Also, the build quality on the Tokina is first rate, and it has worked great on a 5D, 5DMkII, and 1DMkII.