Well, I'm basically into street photography more than anything at the moment and need something light to use with my 5D MK3.
I have a 70-200 F/2.8 II, but that's not a street lens...
I have a 16-35 F/2.8 II and it's way to heavy and bulky for my style
I have a 50 F/1.2 and ditto it.
I have a 50 F/1.8 II and it's pretty light but I just bought a shorty forty now and it's pretty damn near perfect, getting my Canon down to the size and weight (almost) of my Fuji X-Pro 1 (I have 18mm F/2 (27mm equivalent) and 35mm F/1.4 (53mm equivalent) and they are perfect.
So I'm looking for something small, light, sharp and relatively fast.
The 28mm F/2.8 IS is reportedly very sharp out to the corners and is a new lens (like the shorty forty) and a good match to the newer 5D MK3 hardware.
Lensrentals says it's pretty darned good...