This macro telephoto lens appeared in march 2001 on Sigma website, almost 20 years ago. It is a robust and well built macro lens with a minimum focusing distance of 46cm for a 1:1 ratio. I bought mine used in perfect condition, boxed with the pouch, accessories and paperwork for $250.
It is quite a well made lens, made of metal, heavy (1kg), with a well balanced tripod collar and a super large and precise focusing ring. Too bad it lacks weather sealing. The current ratio and focusing distance are displayed in the small transparent window. There is focus limiter switch: full focusing range, 0.61m to infinity and 0.46m to 0.61m. It is an HSM lens so no need for a body with integrated AF motor, and the focusing ring overrides the AF any time.
This lens was made for film, at a time the Nikon D1 (and variations D1H and D1X), and the Canon D30 were almost the only available DSLRs. You had to spend more or less $1500 to own it.
For macro use, it is definitely a great lens if you need a good working distance. The depth of field is VERY shallow at 1:1 ratio, even at F/32, a bit more than shorter lenses actually. That is quite good if you need subject isolation. It sucks if you need DOP.
What is really amazing with this old lens is its optical qualities.
At F/3.5 it is reasonably sharp, the best sharpness is achieved between F/5.6 and F/8, then, sharpness goes down slowly, and diffraction, depending on your sensor, makes things worse pretty fast. But when shooting macro, that is pretty much unavoidable. We need DOP, therefore we need a small aperture, but we lose some sharpness because of that small aperture and the diffraction that eventually kicks in. And then we need LOT of light, hi isos, and fast enough shutter speed (or flash) to freeze movement and avoid motion blur.
Distortion is virtually absent, and there is a very low vignetting only at F/3.5, disappearing at F/5.6.
Chromatic aberration is a non issue on my copy of this lens.
The bokeh is nice at low apertures, I can see it being of some use for shooting portraits, but it is a little less nice when shooting macro to the point of getting ugly at the smallest apertures.
The AF is what you can expect from a macro lens that is almost 20 years old tech: quite slow, even when using the focus limiter. Depending on the camera and subject distance it tends to hunt a lot, and sometimes focus lock is hard to achieve. Continuous AF can be a nightmare, and tracking very unreliable. But when shooting macro that doesn't really matter: I'm using manual focus, and the focusing ring really shines here: it is very accurate, with good friction and easy fine adjustments.
in 2005, that lens became a DG lens, allegedly optimized for digital cameras, with better coating. Likely a marketing gimmick. The current Sigma offer is the 180mm F/2.8 EX DG OS HSM. I tried it and it doesn't really have anything to offer in comparison to the old one, especially when shooting macro.
If you need a macro lens with a good working distance for 250 bucks that lens is the best option.