I have taken hundreds of images with this lens, and every-time it delivers excellent results. What I like most about this Sigma 180 full frame Macro lens is how pleasing the colors and sharpness are right inside the camera. The lens design and glass adds wonderful contrast to the images, and the overall color reproduction accuracy simplifies post-processing. I don't really need to do much to the images once I get them into lightroom, but also to note is the fact I don't generally do much processing to images anyway. My post-processing is to remove noise, I may adjust/balance of light and dark areas, followed by some contrast adjustments and finally adjust the white balance, not necessary as I take a color checker with me on my trips. Sharpening? With today's lens, I rarely do any other than in camera.
How I used this lens: I have two long Macro lens 1. a Nikkor 200mm and 2. this Sigma 180mm. The lens can fill in as a lens that is suitable for a number of photography choirs. Well - Macro, and generally a nice medium telephoto, and also a Portraiture lens, and quite frankly does a good job at all three. I bought mine for macro, and so that is a way if have primarily used it for.
As a Macro lens it rocks - It is a true macro lens. You can rest assured this lens will deliver high-quality performance and sharpness. I believe it to be the best macro from Sigma and challenges both Canon and Nikon glass optically. The images are not as flat and sharp edge to edge as say my Nikkor 200mm, but I do not use the lens for flat objects, I take images of nature so to me it does not matter.
What I liked: Very fine detail and sharpness at the macro range and as good when used as a standard medium telephoto, delivers good contrast and nice colors with the ability to show some bokeh at f/2.8. The lens performs well in the field but be advised this is a "heavy" lens. And when I say heavy the sigma is a FULL Pound heavier than the Nikkor 200mm, sigma is 3.6 pounds, while the Nikkor is 2.6 pounds. The weight does matter, especially in macro when most of your subjects are close to the ground, bending over with the lens and DSLR is a choir;
What is so so: The front lens hood is cheap-plastic and hard to attach securely, Sigma needs to address better hood attachments for all their lens. The case is also crap and cheaply made but does the job. In Sigma's defense, some improvement was made to address the lens hood on their newer lens, but their cases are awful.
Optical Performance: I find this lens to have excellent optical performance with a high degree of quality throughout. I can acquire sharp images both hand holding using the OS or when on a tripod, the results can be amazing, while hand holding the lens is very good but not as good as my Nikkor 105 handheld with VR. Consider this lens is almost twice the focal length, so this lens is near as good but I can not determine a true winner.
Focus: Although the Sigma is speedy my Nikkor 105 has faster focus, but the Sigma beats the Nikkor 200mm in focus speed but not in sharpness. However, unlike the Nikkor 105, the Sigma lens has very minimal focal breathing, this is important for focus stacking. My 105 Nikkor changes size as you focus making it difficult to stack focus. I try to avoid stacking by posting my sensor plane to the subject, but this technique is not always possible, stuff in the way!. So, Sigma focus is fast enough for me as a Macro lens and good enough for medium telephoto general photography. If you use continuous focus mode in your camera, the sigma will clatter a bit as it maintains focus on your subject and for macro small movements will produce a lot of clatter. I do not find it too objectionable, but it is distracting.
Stabilization: Sigma calls it OS Nikon calls it VR. I say both works when used properly, I found no fault in Sigmas OS used on this lens, I get nicely detailed images on the rare occasions I hand hold, when engaged you will hear a click then slight release when the image is taken, nothing too loud. Your biggest challenge out in the field is the wind, and stabilization does not address that - a combination of fast focus tracking and OS will.
Concerns: This lens is heavy. Be advised on this, if you are a freewheeler who likes going around hand holding - this lens will tax you. Please: consider carrying a small tripod with a quick release to allow trouble-free carry or a beautiful Black Rapid strap that helps to remove the strain, but a tripod, in my opinion, is better, when set up with a quick release.
Hand holding> I get about 75% good sharp images. Maybe I am slow and old, on a tripod, I get 90% sharp images - wind plays into this, and no wind = 99% sharp images on the tripod.
Overall Thoughts: Yes, this is a very nice Macro Lens and is what I use it for, Overall I do not find any major fault in the optics or image stabilization both seem to work well. If you are looking for a longer macro, this lens should be close to the top of your list.
Good luck in your Macro work, if you need advice give me a shout in the comments below.