It took me quite some time to actually find this lens, although I wouldn´t call it extremely rare. Probably, most users preferred adding a 28 mm lens to their standard 50 mm lens instead of going for the 35 mm lens.
Usually, the 200 mm prime lens and the 80-200 mm zoom lens is available more often on the used market than this one.
I personally prefer the combination of a 50 mm lens with a 35 mm lens and a super-wide lens between 21-25 mm, but that is just a matter of taste and preference.
The performance of this lens is also very good and superior to the 28 mm lens (especially corner sharpness).