I just received this lens from KEH this past Tuesday, 9/10/13. It was listed in EX condition for $599 and appears to be such, with no scratches I can notice. It came with the hood, lens cover and green case. However, the first time I removed the hood from the lens (it was attached backwards, the way most of us would put one on the lens barrel when we're not using it), a fair number of little black flecks of what looked like dried rubber fell out and onto my shirt. I was sitting in my car doing this, as I saw the box for the lens on my front porch on the way to take my mom to get her hair cut, so I wasn't in a really good position to tak pics of the flecks, but I picked some up and looked at them, and that's what they looked like -- dried rubber -- like maybe the hood had been around the focusing ring a long time and some of the rubber from it had dry-rotted onto the hood and came off when I unscrewed it and removed it... At least that's all I could figure.)
I also looked down through the end of the lens, towards the mount, and it appeared there might be a few flecks of "something" on one of the inner elements, but I couldn't tell exactly what, or where, and I don't see anything, so far, when I shoot a picture -- although I've not yet downloaded anything and looked at it on my monitor. Since I recently upgraded to the Dell 30" 3011 monitor (running at 2560 x 1600, I imagine if there are flecks inside the lens I may be able to see them on this screen).
The lens seems to take pretty impressive macros, though. I've been using the Pentax 100mm macro lens (which sells new for more than I paid for this lens used), and unlike most of you, who seem to be shooting Canon or Nikon, there's NO new option to me for a 150 or 180mm lens, as Pentax doesn't make one and neither does Sigma or anyone else make one in a Pentax mount any more (although I just read on Sigma's site that they'll do a "remount" on some of their lenses for $250, or less, depending on the lens. I DON'T think the big macros are in that league, however, although the desirable 120-300 F2.8 is -- especially since they just came out with a new one and dropped the price on the older one.
The lens seems "very dark" compared with my Pentax macro, which is only a half stop faster. Of course it's about three times as long (and probably five times as heavy). But even when I have either stopped to, say, F4, I seem to get a "brighter" image with the Pentax lens than with the Sigma, I think (although I've not done a side-by-side yet, which I can do, since I have both on two different cameras now (K-5 and K-30 -- Pentax lens is on the K-30). Even when I use the K-5's pop-up flash, which was giving me a decently bright, frozen image in my ILLUMINATED from terrarium (60-watt bulb 12" above surface), I'm getting a somewhat DARK image. That could be, however, that this lens is so long that the popup flash isn't "clearing" the lens shadow -- especially since the minimum focus distance on the Pentax macro is about 12", but it's only about 18" on the Sigma -- which is a MUCH LONGER LENS. Perhaps if I had the lens further back the flash could clear it, but then I'd have more light fall-off, plus my image wouldn't be as big. Ack!
Of course, the other factor -- and I knew I'd be facing this -- is that depth-of-field -- already SHALLOW with the 100mm macro -- is practically NON-EXISTENT with the Sigma 180! I've been shooting little BABY crickets... I'm talking 1/8" long, or so. Even so, their head will be in focus and the rest of their body out of focus -- and forget even being able to tell what their tail IS -- and that's at, like, F11! On the rare occasion that I can catch one precisely parallel to the lens, well -- those occasions are far too rare.
I'm clicking "NO" on reccomendation NOT because I wouldn't reccomend this lens, but because I'm just not sure, yet. I didn't realize quite how old this lens was when I bought it. Looking back through this forum, I see it's at least 10 years old, based on the first post. That may, or may not be old for a motorized AF lens. It hunts as much (or more) than my Pentax lens. I've YET to find ANY USE for the focus limiting. Yesterday, when I went to get more adult crickets for my frogs, I parked by the bait shop (fishing pond) and shot some pics of plants and trees across the pond (100 yards, or so). I was shooting hand-held, braced against my car window. But the car was off, and I shot as fast as 1/750, and I never did get a shot where ANY of the trees, leaves, etc. looked fully in focus. I found that troublesome. However, after I came home I went in my back yard for a while and shot a butterfly on some butterfly bushes in full sunlight, and at least on the camera's monitor, THOSE PICS looked tack-sharp.
So... the jury's still out for me. I'm just glad KEH has a 30-day return policy. I HATE RETURNING STUFF. But I like the option... I'll amend this review one way or the other when I know more.
One more thing... I'm used to "quick-shift" focus on Pentax AF lenses, so it took me a while to figure out how to manually focus on this thing, lol. When I finally did figure out how to switch it to MF, it wasn't worth the trouble. The MF is so stiff it's pretty much IMPOSSIBLE to do fine-tune focusing that way -- at least for me -- unless there's some way to "oil" the focusing ring. I looked on Sigma's website to see if I could find documentation for this lens (some manufacturers KEEP documentation for any and all old stuff... NOT THEM!