I have been using this lens for about a month now, and am generally pleased with it, although it does have some limitations and shortcomings, as does any lens.
Firstly, this is a VERY compact superzoom lens. You can tell that from its specs, but when you actually use one, it is still a surprise how small this is, for what it does. This is a strong contributor to its value as an all-day walk-around or hiking/backpacking lens.
Secondly, it is well-built (for its price), and the controls work well. The autofocus is pretty quick, accurate and fairly quiet, doesn't hunt very often, and is usable in typical indoor lighting. The OS seems to work as advertised. Regarding the autofocus, due to the small size of the lens (shortness), combined with its lack of full-time manual focus (the focus ring rotates during autofocus), if you hold your camera by cradling the lens with your left hand, you have to be careful to keep your hand away from the focus ring. The focus ring itself has fairly limited degrees of travel, and not much friction, somewhat limiting its usefulness. Normally manual focusing isn't much of a use case with superzooms, but due to its close-range focusing ("macro") capability, a better manual focusing system would have been appreciated. The zoom ring on the other hand, is pretty smooth and has a good amount of resistance. I found my zoom ring stiff when new, but it has loosened up to a nice weight. Unfortunately, with this loosening, lens creep is now more of a problem, so I use the zoom lock regularly.
The switches (zoom lock, MF/AF and OS) are well-placed and solid, but maybe a a bit stiff. However, I'd rather have them stiff than too soft such as on my Tamron SP 70-300, where the switches are often accidentally getting moved.
Regarding picture quality, the results are decent. You can find more detailed testing results elsewhere; overall my results are about the same as those of other reviewers. The sharpness is pretty good at the wide-angle through mid-range, getting softer at the longer end. Stopping down to f8, and even f11 at the long end, gives reasonably sharp results. I haven't done a lot of close-up shots, but the "macro" shots I've done seem pretty sharp. The CA seems pretty minimal (I haven't really noticed it so far), and barrel distortion at the wide end is typical, but not too complex, so easy to correct in software. For comparison, I find this Sigma sharper in the middle at 18mm than my Canon 18-135 IS, but about the same at the edges/corners. In the middle focal ranges, it is fairly similar to my Canon 18-135, maybe better at some focal lengths but not as good at others. There are many factors at play here, including the specific focal length, aperture, and focus distance, but I think this Sigma is more consistent in its image quality than my Canon 18-135. When stopped down, the results are generally good at all wide to middle focal lengths, and OK at the long end. At long the end, the lens is softer than my Tamron SP 70-300mm, but that should not be a surprise.
Because I shoot mostly outdoors, and mostly scenic and nature shots, this lens has become the one I leave on most of the time. I only put on a different lens in special cases, such as low light, when I need a more limited depth of field, when I need a super-wide, or when I need maximum telephoto reach (with maximum sharpness). Otherwise, I find this lens to be quite adequate for almost any shooting occasion.
If you are looking for a good-quality superzoom, and understand the limitations of this class of lens, you will probably be quite pleased with this lens. The build quality, good autofocus and smooth zooming make this lens a pleasure to use day-to-day, and the compactness light weight are very appealing. The image quality is nothing to complain about, considering the type of lens, and its reasonable price.