When I first picked up photography, I have always wondered on the viability of spending large amounts of money for a fixed focal prime lens such as this. However, having cycled through quite a few Canon lenses and 'L' series lenses ranging from zoom lenses and other primes, I have arrived in front of this 24mm.
It is no doubt an expensive lens BUT it can also provide you wish capabilities that other lenses cannot. As stated in my pros list, it has a very thin depth of field when used wide open and that really adds punch to your photos by making your subject 'pop' out more (when focused properly). This will definitely appeal to certain photographers and in my case wedding photos.
In my humble opinion, you need a full frame camera to really appreciate this lens. For wedding event photos, in many instances we would like to include as much of the scene as possible to 'tell the story' and we usually lose the dramatic effects from blurred/out-of-focus backgrounds when using the wide angle zooms with apertures limited to F2.8 and above. With this lens coupled to a full frame body, it allows the photographer to maintain both the coverage of the wide angle and the background blur.
As such, if you are looking for a lens to take photographs at F4.0 and above at 24mm, then I believe you are better off looking at the other canon zoom lenses (e.g. 17-40 F4L, 24-105 F4L, 16-35 f2.8L).
In terms of picture quality, it is very good wide open BUT not earth shaking. At F1.4, there are signs of purple fringing and definitely not as sharp when used a few stops down. However, it is barely noticeable unless you scrutinise under pixel peeping.
All in all, I have no reserves in shooting with this lens wide open and having printouts of A3 or A2 size.
I hope this gives an idea on the usage of this lens and assist anyone who is considering this lens to determine whether it is the right lens for them before making the big jump.