From Ken Rockwell's review:
There are three primary reasons to own this masterpiece:
1.) Used at larger apertures, gives a unique look which cannot be duplicated any other way. There is no way in software or with other lenses to duplicate its precise combination of artistically pleasing falloff and its unique rendering of out-of-focus areas. If you want this effect, you need this lens.
2.) For ultra-low light, f/1.0 is 1/2 to 2/3 stops faster than common f/1.2 lenses. In fact, this f/1.0 lens as about as much faster than an f/1.2 lens as an f/1.4 lens is faster than an f/1.8 lens.
While the ultra-high ISOs of today's DSLRs let us capture anything in any light hand-held with slow f/2.8 or f/4 lenses, we still can't stop action at 1/125 in near darkness unless we have a lens this fast. High DSLR ISOs let us shoot indoor and night sports that are dimly lit, but if we're shooting where there isn't any supplemental lighting, we need the f/1.0 to stop action in the dark.
3.) Subject Isolation: no other 50mm lens can isolate a subject from its surroundings as strongly as can this lens. If you can't control your background when shooting outside your studio, this lens lets you make the background go away.
Today when every other amateur photographer is probably using the same camera (or better) than you are, one way to stand out and win more jobs is to master a lens like this and give your images something that weekend amateurs can't copy.
For instance, shot at f/1.0 in daylight, it so limits the depth-of-field of an otherwise normal image that it really does look like a shot made either with an 8x10" camera wide-open, or sometimes as if the subjects are just miniature models. (It seems that way because we're used to seeing limited depth-of field usually only if something is very small and therefore shot close-up.)
