I haven't used the 28mm for astrophotography yet. I'd like to get a wide angle with a slightly larger aperture, and one that's a bit sharper wide open. I do love the Pentax-M 28mm F2.8, I used it almost exclusively on holiday in New Zealand.
As for software, the main ones I use are Star Trails (for making star trails, obviously), DeepSkyStacker which has a really good range of stacking techniques, and also GRIP which is really good at rotating and warping the images. So for the below image, which was taken with the 18-55mm kit lens, I found that DeepSkyStacker did not rotate and align the images well due to the bad distortion at wide angle on this lens. However, GRIP has a "warp" mode which doesn't just rotate the images but applies a warp which ensures each star is aligned across the entire the frame. So I used GRIP to warp the images and save them, then DeepSkyStacker to stack the images (without rotating them).
I wish I'd used a higher ISO in this shot. It's about 100 exposures, F3.5 and ISO1600 with the 18-55mm kit lens @ 18mm