First off, in a previous life I worked as a portrait photographer. Nowadays it's more of a hobby, but I spend a significant amount of time involved in it, and have travelled to many countries specifically to take pictures. I know my way around a camera, but don't consider myself a pro.
I learned photography 'old school' in the film days; pick up a camera, put a 50mm lens on it, and shoot about 10000 pictures, then you can change lenses. Like a lot of photographers, the 50mm lens has a special place in my heart.
This still applies on the full frame 5D which I shoot with. I don't find the 50mm length useful on crop sensor bodies.
So in looking to fill this 50mm gap, over the last few years I have used the 50 1.8, the 1.4, and the 1.2. The 50 1.8 is actually pretty fantastic for the price. The focusing and build quality, along with the kinda crummy bokeh later led me to the 50 1.4 which I have been pretty happy with. The 1.4 has good bokeh, decent build quality.
What led me to try the 50 1.2 was that the 1.4 seemed to miss focus often enough (maybe 2/10 shots) that I ended up throwing away some photos that would have been quite good had it hit properly. Though the price is quite a bit steeper, I figured the 1.2 was probably going to be worth it, as I have been extremely impressed by most of the L lenses from canon.
I hate to say it, but I am pretty disappointed. I have tried 2 different copies now. Don't get me wrong, it's not a bad lens. But I ended up returning both copies soon after getting them for a few reasons. First, there's the well-known backfocus issue that I will not go into except to say, both of my copies backfocused a bit, but not so much I couldn't live with it. It was about 2-4 mm.
The second reason I returned them is that it misses focus nearly as often as the 50 1.4. And don't mistake this for operator error in a very shallow depth-of-field. My real-world shots along with tripod test shots often were out of focus for no apparent reason. Taking 3 shots without moving the camera or subject would only yield 1/3 in focus shots. (and none of my other lenses have this issue so it can't be blamed on the camera: 17-40, 24-105, 85 1.8, 70-200/2.8, 135, 100-400).
Third, and worst of all, aside from a small improvement in bokeh at 1.2, there is very little difference in most comparison shots between the 1.4 and the 1.2. I suspect that the 1.2 would hold up better in flare situations and other times, but in my real-world shooting, my 1.4 is sharper and has the same color and contrast rendition. I'm not a photojournalist in Somalia so while the build quality is nice, I don't forsee breaking my 1.4 anytime soon and if it does I can pick up 3 more backup copies for the same price as the 1.2.
I really hope Canon comes out with a 50mm that befits the L standard someday. I'll be the first to buy it and try to hide the receipt from my wife. But for now, in my opinion, it's just not in this lens.