I got this. It's a bit of a sleeper lens for the type from what I am reading. I've been seeing it for like $85-100 unless it was a lens with an issue. $120 new if you can find it. It's been discontinued. I got it for $25, bargain lens. Could not resist that. I've been wanting an AF lens in that range and the reviews were pretty good.
There are several version of this lens. I, II, and III, of the 3 the last is the only one people seem to like and that's the one I got. It was improved a lot apparently after the first two. The first two were apparently clunkers, but III I've now seen quite a few people saying that they like it so I thought I'd give it a shot. I've been looking at them for a while but this was the first one I saw that I really thought was a bargain.
I trust KEH. I have bought 3 AF lenses from them, all ranked bargains, and they're all great. Not a clunker among them. This lens? Cost me my junk food and a movie out this month, but that's okay. I hear The Lone Ranger sucked anyway, and I can live sans the Big Mac and FF.... :P
Sigma 28-105mm F3.8-5.6 UC III Lens Reviews - Sigma Lenses - Pentax Lens Review Database