I bought an 11-24 in early 2016 and have used it for Commissioned and personal work since then. In a nutshell, it's an incredibly sharp, heavy lens, that is more of a specialist item than say a ubiquitous 16-35 or 24-70. But when you need that extra width it's a tremendous asset. I'd strongly recommend trying one first.
Early Trial - Hiring One
I first tried the lens shortly after it was released in 2015. I was on a trip to Tenerife to photograph the milky way with a friend who had hired one. I used it alongside my 15 mm f/2.8 fish-eye (Canon) and Samyang 14 mm f/2.8. While f/4 might be considered slow for astrophotography the 11 mm wide angle meant slightly longer shutter speeds before star trails became evident. It certainly wasn't a limiting factor.
The above image is almost full frame, the only piece of editing in Lightroom that made a noticeable difference was applying the lens profile. At f/4 vignetting is obvious and quite heavy. By the end of the trip I'd seen enough to know one would be very useful for a lot of my commercial work, particularly when you are trying to photograph a subject or environment that would normally be difficult to capture.
It's heavy (1180g, so around a third heavier than the 16/35 f/2.8L III and double the weight of the 14 mm f/2.8L II), it zooms from 11 mm to 24 mm (126° diagonal angle of view) and has a widest aperture of f/4. It is incredibly sharp and handles distortion / aberrations well. Additional technical bits and pieces can be found on the Canon site.
The first task when using it was to zoom it all the way out to 11 mm (why wouldn't you!). I was surprised how much wider 11 mm was compared to say 14 mm. 3 mm doesn't sound a great deal but in ultra wide angle terms every single mm makes a big difference.
This is one example of how different the images can be taken with this lens. The Virgin 747 was on very short finals to land at Manchester and was literally above me. It took me half a dozen attempts to frame other aircraft properly before I got used to the rather weird sensation of looking through the viewfinder and seeing what appeared to be an aircraft much further away than it actually was.
Again at 11 mm the front of the lens was very close to the front tube. The lens offered a totally new and dynamic perspective on the Tornado.
This image was taken as part of some commissioned photography for a client providing training services to the RAF.
The 11-24 / 5DsR were mounted on an L bracket which was attached to my Arca Swiss D4 geared tripod head. The tripod was positioned in the rear seat of the Tornado - it took me a few minutes to set up. We set the simulator to create the effect of descending onto the island. At 11 mm on live view the scene looked superb. At 14 mm and beyond I lost most of the cockpit.
0.8 sec f/4 ISO 800.
Comparing 11 mm to 24 mm
The images below were taken in the BAE Systems chalet at Farnborough 2018. While not a perfect comparison they give an illustration of the difference between 11 and 24 mm. While my position clearly changed it is hopefully evident how much more 11 mm offers. Note for example the large empty space in the foreground of the 11 mm image and how much stronger / pronounced the lines and shapes become. At 24 mm things tighten up with the aircraft fuselage more prominent. The versatility of the lens to deliver images of such varying perspectives is so useful. Both images were taken using a tripod.
I generally don't go wider than 24 mm with my landscape images. If I did need something wider I'd use my 17 mm TS-E, or stitch an appropriate number of vertical images. Of course the 11 -24 is capable of delivering impressive results but I can't see that many uses of it below 16 mm for landscapes. In any event a crucial point to consider when thinking about it for landscape would be the use of filters. With say a 16-35 or 24 - 70 a 100 mm Lee filter system can be used across both lenses (and others), indeed for my 17 mm TS-E I only needed a specially designed adaptor ring to allow my existing range of 100 mm filters to be used.
Filters for the 11 - 24 mm require an entirely new set of "larger" filters (150 mm), and a new adaptor and filter holder - this is the Lee SW150 system. To effectively repeat a similar set of 100 mm filters I already own would require around £800 of investment. I decided it wasn't worth it.
Where Have I Used It?
Looking through my Lightroom catalogue I've used it in the following areas. Aircraft simulators, office interiors, boardrooms, astrophotography, civil aviation, military aviation, corporate product work (in use), exhibition stands, museum / church / cathedral interiors and city scenes. It has become such a useful lens for my commercial work offering perspectives that are otherwise difficult to achieve. For my own personal work it allows me to create unique images with the ultra wide angle perspective emphasising lines and shapes resulting in more dynamic compositions. You quickly realise that you've got to be very, very close to a subject when using it at say 11 to 14 mm - subjects are easily "lost" if the frame isn't filled.
This lens is an incredible piece of optical design and engineering. It is built like a tank (and feels like it) delivering superb quality with little distortion across the focal length range. Vignetting quickly disappears (like most lenses) after they are stopped down by 1 stop (to f/5.6 in this case). I would class it as a specialist lens that is really a tool for specific circumstances. If those circumstances are what you photograph regularly, either personally or commercially, I'd highly recommend it. On the other hand if you have a special one off need then I suggest hiring one and if you really like it buy one subsequently.
Any questions please ask.