I have a couple. I just recently got the 24mm f2 and I have had a not-quite-right 105mm f2.8 macro. Both have Vivitar equivalents. I also have a couple of Vivitar-Kirons and one Panagor-Kiron. The 24mm has a reputation for oily aperture blades and I had to fix mine. Otherwise I think they're great. I might get a couple more.
Not the Vivitars made by them, but an actual Kiron brand on the lens. Ever seen that?
How are they?
I have Kiron 105mm/F2.8 1:1 macro which I bought new around 1986. It is exceptionally well built lens with precisely machined all metal body and retractable shade. It works best on K20D, very sharp even at F2.8. Great example of optical engineering.
I have a Kiron 28-105mm that I got for a walk around lens for my ME Super. It works well for that. Build quality is good and the lens is plenty sharp. It does occasionally suffer from some CA's in the right situations, and zoom creep. But not too bad. I think I only paid $10.00 for it, so the price was right. It has a "macro" setting too which can be useful.
A sample shot of an old water cistern. Shot on some expired Fuji Superia 200.
I also have a Kino Precision (Kiron) 24/2, which I used many years on film cameras and was pretty satisfied. The aperture is still snappy and free of oil. I never used it on a DSLR, though, since I also have the FA 24/2.
We got into this discussion in another thread and it turns out what I thought was a Viv 70-210 f4 Kiron made lens is actually a Kiron branded 70-210 f4 lens. From some of the shots I've taken, it takes some really sharp images.