Make the rest of the world go away
If you're a candidate for owning this lens, you'll probably know it. It's definitely a specialist. If you have a need for shooting in very low light, or you want to see a focus difference between an earlobe and an eyelash in a tight close-up, you'll enjoy your time with this lens immensely. Its focal plane (the slice of space in your camera's view that's actually in focus) at f/1.2 is almost paper-thin. While the in-focus parts of your portraits will be exquisitely sharp, your backgrounds will melt away to put your subjects into a world of their own. Wedding photographers will adore this lens, as it creates a sense of poetry and emotional depth in portraits that's hard to equal.
This lens is large and not exactly light. Its framework exists to support the sizeable precision-machined glass elements necessary for a lens this light-sensitive. But the attention paid to the optics shows up in every picture, with outrageously sharp reproduction from edge to edge, even at f/1.2.