With focal lengths from 28mm to 200mm (and sometimes more), and everything in between, you can frame a scene exactly as you want it, without having to change lenses or move closer or farther away. This can mean the difference between catching that fleeting moment and missing it while you're frantically trying to swap lenses. Fewer lens changes also means less chance for dust to enter the camera and settle on your sensor.
Many all-in-one zooms will focus closely enough to produce a 1/4 or even 1/3 life-size image at the image plane, great for frame-filling shots of flowers and bugs. That's not true macro (true macro lenses focus close enough to produce life-size 1:1 images at the image plane), but it certainly adds to the versatility.
All-in-one zooms (those with a zoom ratio of 7:1 or greater, for our purposes here) also cost relatively little, certainly much less than a set of prime lenses or shorter-range pro zooms that provide the same focal lengths. And, if you use filters, a "superzoom" also can save you money there, too; you don't need to buy filters to fit several different-diameter lenses.
So, what's not to like?
Well, for starters, the all-in-one zooms aren't quite as sharp as short-range zooms or prime lenses. That's because each focal length has its particular aberrations, distortions and vignetting that need to be sorted out, and it's not possible to completely correct all of them for an extensive range of focal lengths in a single lens. Corrections that help at short focal lengths can make things worse at longer focal lengths, for example. A big-range zoom lens is a compromise in terms of optical performance.
This doesn't mean all-in-one zooms aren't good, just that they aren't as good at a given focal length as a prime lens of that focal length or a pro short-range zoom that covers that focal length. Computer-aided designs and exotic elements (especially extra-low dispersion and aspherical ones) do a remarkable job of producing surprisingly good performance throughout the focal-length range.
For general photography, especially for a photographer who wants to travel really light or is on a tight budget, an all-in-one zoom is an excellent choice. But, if you use a high-megapixel camera and specialize in huge prints, you'll probably want to go with higher-end lenses.
Superzooms & Sensor Formats
|Full-frame DSLRs have image sensors the same size as a 35mm film frame, approximately 36x24mm. Most superzooms for these cameras are in the 28-200mm and 28-300mm range.
APS-C sensors used in many DSLRs and mirrorless cameras are smaller, measuring around 23.6x15.6mm. Due to their smaller size, these sensors "see" less of the image produced by a given lens; a lens on an APS-C camera frames like a lens 1.5X that focal length when used on a full-frame DSLR. Put a 100mm lens on an APS-C camera, and it frames like a 150mm lens on a full-frame camera. Thus, superzooms for APS-C cameras tend to be in the 18-135mm and 18-200mm range (equivalent to 28-200mm and 28-300mm lenses on a full-frame camera, respectively).
Micro Four Thirds (MFT) mirrorless cameras (and Four Thirds System DSLRs) use even smaller sensors, measuring 17.3x13.0mm and having a diagonal measurement half that of a full-frame sensor. MFT sensors have a 2X focal-length factor: a 100mm lens on an MFT camera frames like a 200mm lens on a full-frame camera. So, superzooms for these cameras tend to be in the 14-150mm range (providing framing like a 28-300mm zoom on a full-frame camera).