The Pentax 28 has 11 mm of shift, the Arax 28 has 11, and the Rokinon 24 has 12.
In contrast, "Composition Adjust" offers sensor shift on the K-5, K-7, & K-1 but can only do ±1.5 mm vertically and horizontally (or about 2 mm diagonally although there's not many compositions that would benefit from diagonal shift). The nice part is that you can use it with any lens and it's free. (I'm not sure how much shift is available on other Pentax models.)
MikeSF created a excellent thread on this topic (
Composition Adjustment/Sensor Shift...are you using it? - PentaxForums.com) including this example of the effects of it:
Shift has more noticeable effects on full-frame and with wider-angle lenses.
SpecialK has a good list of the uses of shift and tilt. And although the shift effect can be done in post (with perspective correction or keystoning tools) it degrades resolution and affects framing. It's sometimes more effective the create the picture you want in the camera than hope what you took can be corrected the way you want. You can also create exaggerated perspective by shifting in the "wrong" direction and that's an effect that can't be done in post without starting with an image from a much wider-angle lens. The tilt effect is much harder to simulate in post -- bringing a tilted plane into focus would require focus stacking.