This is a remarkable lens. It's very heavy, and wider than it is long. Some would say it's the best portrait lens you can get your hands on. It is incredibly sharp even wide open, and the wide aperture blurs the background beautifully. In fact the depth of field at f/1.2 is so shallow, that you might find yourself having to stop it down a little, to f/1.4 say, just to get a subject's entire face in focus!
With the lens wide open, it's possible to shoot in near darkness or candlelight. It's a popular choice for photographing concerts and stage shows. The autofocus isn't the fastest in the world, so you wouldn't want to use this for fast-moving sports - the 85mm f/1.8 would be a better choice there - but it's a very addictive lens all the same.