I rented this lens for a week from the excellent www.rentalens.ch website (highly recommended if you live in Switzerland). I had read so many things about this baby that I wanted to try it out before possibly buying it. Glad I rented it first, because to make a long story short, I'm not buying it (for now... read on).
This little summary of my experience is not a full blown review. For that, I let you find more complete reviews of the Canon 85/1.2 lens. I only want to share my impressions of this overall excellent lens.
The first thing you notice with the Canon 85mm ƒ/1.2 lens is its weight! Oh man, it weights a ton (1.025 kg to be precise). You better be a strong man to carry this puppy around (my elbow is still sore after having used the lens exclusively this week-end). Also, the size is just humongous. To give you an idea, it's about the size of a big grapefruit. My personal camera is a Canon 450D (aka Rebel XSi or Kiss X2), which weighs less than half of the weight of the lens (475 g). To balance it out, I have the grip with the 2 batteries and I put my camera strap vertically (1 side on the grip and 1 on the camera). With such a heavy lens, the camera balances out neatly. I would definitively recommend you get the grip anyway if you swap the kit lens for any serious glass on the entry level Canon cameras (3 or more figure models like 600D or 1100D). If you only have a lightweight body camera, it will definitively feel front heavy with the 85mm/1.2.
The next thing I immediately sensed was that it was too tight. By that I mean that it was too close to be a general lens. With the 1.6x crop factor of the Canon 450D, the 85mm is actually a 136mm equivalent on a full frame sensor. It's still OK for portraits, but you certainly have to have a great deal of space between you and your subject. The minimum focusing distance of the Canon 85mm is 0.9 meter anyhow. Anything below 2 meters was simply too close for me.
This lens is a portrait lens by definition. For portraits you want your subject to be in focus and the rest out of focus, so I wanted to shoot mainly at ƒ/1.2 apertures. The problem is that at ƒ/1.2, your depth of field (the sharp zone in the picture) is very small. You really have to focus sharply on the eyes, as even the temples will be out of focus! The way I work is that I usually use the center Auto Focus (AF) point and then recompose. When you do that, you have to be very careful to keep exactly the same distance to your subject and not move the camera forwards or backwards, or the eyes of your subject will not be sharp anymore. That's just a question of practice and I could get about 95% of my shots sharp like that.
Where it gets extremely hard is when you're photographing moving people, like kids. In that case, you need to rely on the AF of your camera and not try to recompose. In that case, it's probably better to use all AF points than only the more sensitive central one (which on the 450D works by detecting contrasts both horizontally and vertically). But, then you don't have control of which AF point will be used and you have to hope your camera chooses the right one! Unfortunately, I didn't find that very reassuring, and in general, I can really say now that I've reached the limit of the 450D's auto focus system (which is poor anyhow, but I thought it might have worked better with a 1.2 lens). So focusing is a real issue here. You can see the problem in the 2 pictures above. The one with the lady's closed eyes is in focus, and on the other picture, the focus is more towards the ears (on the temples).
Another issue I discovered is that I had too much light. Outside in the sun, shooting on Aperture priority (Av) of ƒ/1.2, ISO 100, shutter speed would often be of 1/4000 second. That's the maximum speed of the 450D. If the sun was really bright, you could easily be limited by that 1/4000, as you'd need 1/8000th of a second. The solution is to go a bit in the shade. Not a real issue in real world, but one to be aware of and check regularly in the viewfinder if the 1/4000 is blinking.
The photo quality overall was really excellent. I did notice some chromatic aberration (purple fringing) at ƒ/1.2, but from what I read it's due to focusing issues; and from my real-life experience, it's not so disturbing.
In conclusion: I would highly recommended the Canon 85mm ƒ/1.2 for portraits if you have a serious camera (full frame with good auto focus capabilities). For crop sensors (like APS-C), I would recommend you look for a shorter focal length prime (maybe the Canon 50mm ƒ/1.2 which I have yet to test).