Summary: This lens is one of the best I ever owned. I create master portraits with this. See the example I have uploaded. This lens is good for events as well. The wide angle enables you to do close courtiers shooting.
Conclusion: I chose this over both 17-40 and 16-35 because I wanted some leeway above the f/4 aperture of the former and the latter (mk I version) was $300 more. So I found a guy who was selling a very decent copy of this lens and... it's been with me for over 1.5 years.
Pros: + great build; + fast AF; + good sharpness, at least from about f/4.0 but usable wide open as well.
Cons: None really. I wish it had been sharp at f/2.8 but for the price, I can't complain.
Conclusion: I picked this great copy off CL a few months ago and haven't been happier. I ended up switching from a 24-70L to the 17-35 and by doing so opened up more possibilities. The USM works great and the colors are nice.
Pros: Light weight, good focus speed, great color and contrast.