(Lens released 2009. I used it for a few years. Like most of my Canon lenses, I have sold this one... Very sadly in the case of this one. Review written 2017)
This lens has the best image quality of any lens I have used… and possibly any lens I will ever use. Sadly, I had to sell this lens as I couldn’t justify keeping it – I wasn’t doing enough landscape photography. Of all the lenses I had to sell, this is the one I am sad about.
Image quality: the best.
Bekeh: the best I have seen – smooth, even, round. Similar in smoothness to the 70-200 f2.8L IS USM.
Practical? NO. This is probably the least practical of all lenses: no weather sealing, no autofocus, no image stabilization. Plus it has these knobs sticking out the side, which make it look weird. It’s a complicated scientific instrument and I wouldn’t want to damage it.
Manual focus: excellently smooth ring, but as with all Canon focus rings, it is not very firm so it could be accidentally moved if you were holding onto the ring (as opposed to some Zeiss rings which are very firm).
Aside from all that, this lens has the unique abilities of a tilt-shift lens and therefore has the ability to make your landscape shots unique from those shot by other people using regular lenses. Most people would say this is a tripod-only lens, but I disagree. I used this lens handheld: (I mostly used tilt, not shift) I would set the angle of tilt, then hold the camera up to my eye and adjust the focus ring until my subject was in focus. That worked fine for me, and therefore didn’t take much setup time. For example, the banner of my "Locations" page, was taken with this lens handheld.
I wish I still had this lens…
The best I have ever seen. I don't have much else to say in this regard. I didn't notice any chromatic aberration and there was certainly no softness. Sharp from corner to corner and far exceeding the 17-40 f4L, for example, in all aspects of image quality.
I wish I still had this lens. Weigh your options very seriously before selling this lens - you are unlikely to find a lens with better image quality. On the other hand, it's a pretty expensive lens, so you have to plan to be doing a lot of landscape photography to justify buying this lens in the first place. The newer Canon 11-24 f4L will likely weigh in on your decision - but, new, that lens is at least $1000 more.