Context: (Lens released 2003. I used it for a few years. Like most of my Canon lenses, I have sold this one. Review written 2017)
The full name of this lens is the EF 17-40mm f4L USM. As you can see, it doesn't have the "IS" designation so it is not image stabilized. This is an old lens and some of Canon's newer wide-angle zooms have IS. All L-series lenses have pretty good design including this one, and like most L series lenses, this one has weather sealing. To me, that's a pretty important thing, and the item that is missing on Sigma Art lenses. Solid construction, amazing autofocus, and weather sealing - functionally, this is a great lens. Image quality is another matter.
This remains to this day, in 2018, the fastest focusing lens I have ever used. Accuracy was excellent too. No complaints in this department.
At one point I considered replacing this one with a 16-35mm 2.8L but the image quality was close enough I just kept this one for the time being. Centre and mid sharpness was excellent - made full use of a 20 MP sensor, but at the corners it turns to mush. Chromatic abberation (purple fringing on features of high contrast) was also a problem with this lens.
I took this lens through hell and back - environments I didn't even want to take a digital camera so I used film. This is a great travel lens, especially if complimented with a 50mm 1.8 or 1.4.