First of all, this lens is relatively HUGE. With that size, goes a considerable mass. In isolation, one wonders how it could be useful, with such a large mass/weight. However, in use on my EOS 5D/BG-E4 grip, it feels remarkably 'balanced'. I wouldn't like a day's shooting with this lens without the BG-E4 though...
As to the images it produces, it's hard to explain when you first see the first 'roll' from using this lens. Initially, you almost feel slightly underwhelmed, thinking 'Is that it?', especially when the lens is bulky, heavy and extremely slow to focus (compared to other EOS lenses).
But after further use, 'the penny drops'; you have to learn to use this lens properly. This lens literally defines the meaning of 'sharp', which takes some getting used to. I think we get used to looking at images that sometimes lack clarity and 'bite'. This lens, however, is almost brutal in it's uncompromising ability to pick out every detail, AND every flaw. It's a bit like watching HDTV after a life of standard definition.
Also, the depth-of-field at f/1.2 takes some getting used to. Ever had a portrait where the DOF is so limited that a subject's pupil is in focus (as it should be for a portrait), yet the eyelashes on the outer aspect of the upper eyelid are out of focus? Me neither, until I started shooting with this lens.
Saying that, and with all of the above, this lens has produced some of the most beautiful images I have ever produced. OK, unless you learn to use this lens properly, the 'keeper ratio' might be a little low, but when you get that image that literally blows you away, you forgive all of this len's shortcomings, shortcomings that are in reality a result of the use of incredible, superlative technology and ability in the hands of a mere amateur.
So does this lens qualify as the 'holy grail' of Canon red-ringers? Well, IMHO and with my very limited experience, it may well have the potential to do just that.