It used to be that the main game in town for a compact SLR was the D40x and the Nikon 18-200 superzoom. Now the market has caught up and both Canon as well as Sigma and Tamron have shipped superzooms. So if you have a subframe camera, what’s a good choice? Well, unfortunately, if you are a Canon owner, it is filled with compromises. Looking across all of these, is a hard choice. Tamron is out because of build quality and autofocus issues. I’d probably give a slight nod to Nikon and Canon just because they are F/5.6 rather than F/6.3 at the high zoom, but they are more expensive. So Sigma if you are on a budget.
Sigma 18-200 F/3.5-6.c DC OS HSM was the first 11x zoom for the Canon. I got one. It wasn’t super expensive. I would agree though that it has its compromises. DPReview found that the main issue was that it wasn’t sharp at 80mm or so, so ironically, it does well in wide and highly zoomed, but not in the middle. It has relatively low distortion (compared with the Canon, see below) and chromatic aberration. It’s optical stabilization isn’t as good, although for me having four stops of stabilization hasn’t really been an issue at that kind of low light, it is the subject, not the photographer that is the problem. There just aren’t that many things that are that still when it is dark. The other disadvantage is that if is F/6.3 at 200mm, so is really only good in bright daylight.
Tamron AF 18-270 F/3.5-6.3 Dii II VC LD Aspehrical (IF) MACRO like all these has an amazingly long name. It is even more telephoto and has vibration reduction just as good as the Canon or Nikon. The main issue is that it is slow in autofocus, so in the real world not a great choice. And there is geometric distortion and chromatic abberation as well.
Canon EF-S 18-200mm 1:3.5-5.6 IS was finally introduced last October to compete with the Nikon. It really does have some compromises. Wide angle was mediocre because of barrel distortion, chromatic aberration and softness. You can cure some of these except for softness with their supplied software. At zoom, it has distortion at 50mm and chromatic aberration at 200mm. Compared to the Nikon, it is about the same in quality. With Nikon more sharp at 18mm and Canon at 200mm and doesn’t have Nikon’s poor performance at 135mm.
Nikon 18-200mm F/3.5-5.6G IF-ED AF-S VR DX was the lense that started it all in the superzoom category. Certainly it is way better than the el cheapo lenses. Like all these in this category the idea is decent performance but make it lightweight so you might actually travel with it. The main issues are distortion and softness at 135mm.