This lens used to be my favorite. It focuses very quickly, quietly, and accurately. The focus and zoom rings are sized well and turn correctly. The movement is completely behind the filter threads, which is nice for those who intend to use filters. Filters attach to a fixed lens barrel, or attachments can be purchased for a square or rectangular filter. This attachment is best purchased for those intending to utilize GND filters. I like the 77mm filter size because it makes it pretty easy to use the same filters as other lenses. When a UV filter is applied to the front, this lens is completely weather-sealed. The lens resists flare quite nicely, and colors and contrast are good. Chromatic abberation is controlled well, but you will see some in the corners when you're wider than 24mm. There is very little vignette, but you can only use one round filter at a time if wider than 24mm or else the outer part of the filter will show in the frame. Some distortion should be expected. I see a moderate barrel distortion at 17mm and a mild pincushion distortion at 40mm. I find that 24mm is the focal length where distortion is minimal.
Compared to the 16-35mm f/2.8 lens, this one is much more affordable. It has the same minimum focal distance of eleven inches, and is a few ounces lighter than the 16-35mm.
I will complain that the lens is not as sharp as I would expect from an "L" lens, even when stopped down to f/11 or smaller. Also, the EW-83E lens hood does almost nothing for the image quality in my experience. It is very wide and very short, and really doesn't even protect the lens much. I often have to use my hand to block the sun flares even when the hood is on.
As far as uses go I would recommend it as a general-purpose lens if you can deal with the unsharp factor, especially if you are using it on a crop-sensor camera. It is mostly used for landscapes and architecture. I have used it for portraits on occasion, but I often have to sharpen the image to feel satisfied with the outcome - especially if using it at f/5.6 or smaller. Also, it is not good for head shots because of the distortion, as it is an ultra-wide lens. It's really best to just invest in a portrait lens if you are photographing people.
San Diego - with EF 17-40mm at 17mm on full frame camera