Sigma 100-400

Any body know what the sigma100-400 lens is like as ffords have a second hand one in

Do you mean 120-400mm?

They have recently released a 100-400mm but not in Pentax mount.

They've a 100-300 and a 120-400.

Sorry 120 -400

Great lens, even on a K-1 full frame.
Fast AF, in-lens shake reduction (which i keep turned off), bulky, very good image quality for a long zoom.
To be used with the K-1, i recommend to send it to Sigma, to have the bayonet replaced and (more important) the firmware updated.
I also own a Tokina AT-X 80-400mm, that is not at the same level. AF is slower and image quality is good enough only at central diaphragms.

If you decide to buy it, check the objective at different focals and focusing distances. The two rings should be smooth, and you should see no vignetting even at the extremes. Modern Sigma zooms are quite good, sometimes exceptionally good, but they are also very fragile. The plastic "guides" inside the barrel can be bent by a minor knock that leaves no external damage. If the front of the lens hits an hard object, the objective can easily get damaged. Most people never encounter such problems, cause they take good care of their equipments, but some people damage their lenses in a rather subtle way, and then they sell a lens out of whack...

cheers

Paolo