So... first the bad news.
It is very slow to focus, even when doing it manually. It uses Electronic Manual Focusing (EMF) which means that you have to have the camera body on in order for the focus ring to change. I personally don't mind it, but it is just different from most lenses. It is not a good lens for sports that require a fast focus-tracking, and it takes a while for people to learn how to use it without back-focusing.
The lens is very heavy and fat. However, I have come to appreciate the weight of the lens because it is easier to keep it still for shots with longer shutter speeds.
This lens is the KING of flare! I have come to appreciate this as well, because it adds to the art of some of my shots, but if you don't want flare in your photograph, be careful! The lens hood actually does help alleviate the flare problem (for sun but not other lights). The hood wobbles a bit, but it is easy to get on and off the lens.
Let's talk about FLASH! This lens does not provide the camera with distance information if the flash is used on E-TTL. You can still use E-TTL, but it is much harder to optimize.
The price tag is outrageous for this lens. Quite honestly it is ridiculous that you can purchase four 85mm f1.8 lenses for the price of one 85mm f1.2. Come on, Canon!
Now for the good news...
Like I stated before this lens is SHARP! This is very important for my portraiture. In fact, I would recommend this lens above any other for wedding or fashion photography.
It is also the fastest EF lens Canon makes! If you open the aperture all the way, the background disappears completely, which is great for minimizing distractions. (Of course the depth of field is very thin when the lens is open that wide.)
I use this lens for landscapes too!! I love being able to create a dreamy telephoto-esque feel with this lens on landscape photographs by shooting wide open. (Ordinarily, this is a portrait lens.)
I can hand-hold a shot with this lens wide open in low light and get great results.
The 72mm filter size is convenient as it is a common size.