Indeed, we shouldn't neglect this remarkable lens - one of the precious few apochromatic lenses made for K mount.
It has been a favorite of mine for product photography:
Pentax K5IIs - Sigma 180mm f/3.5 APO EX
Burson Supreme Sound V5i Op-Amps
Pentax KP- Sigma 180mm f/3.5 APO EX
Monoculars: Carl Zeiss Vs Pentax.
It is also indispensable for photographing flighty and elusive insects.
Pentax K5IIs - Sigma 180mm f/3.5 APO EX Pentax K5IIs - Sigma 180mm f/3.5 APO EX - Pentax AF160FC ring-flash (Another rare item)