I've read all the professional reviews on the internet before deciding on the lens type, and I'm not going to copy them. There were 2 disappointed ones (one Christopher Frost Photography on YouTube, one semi-disappointed in TheDigitalPicture), and one of them reverted after this (Christopher Frost Photography, when doing comparison review of 6 lens, put these in 2nd place above just all the others, doing them a better justice). Most other reviews, including PCMag, ePhotozine and LensTip, were giving it high marks for its segment, some being positively surprised by the image that can be produced by such versatile lens. I've tried them on a relatively inexpensive Canon EOS 700D and I don't think I can be much more satisfied for what I was looking for. These deliver very nice macro shots, the zoom range is huge for such small and light lens, and the quality of the resulting pictures is far above what's needed for vacation memories, while the versatility allows capturing everything between a wide landscape and a bird far away.
I already owned a Nikon 18-300 mm zoom lens which is pretty good. It's a bit soft at 300mm though and using it at 200mm gives similar results when enlarged in photoshop. I thought I'd buy a Sigma 18-200mm in the hope that it might be sharper at the top end.
The Sigma pros are that it's significantly smaller and lighter than the Nikon. The lens quality is good and it would make a good all purpose standard lens.
The Nikon pros are that it's slightly sharper than the Sigma, and it also makes a good all purpose lens.
I'm afraid we just have to accept that with such a wide ranging zoom we're not going to get pin sharp images at the top end.
I haven't yet decided which lens I will use as my standard - we shall see!