There are pluses and minuses to using kit lenses. So don't get me wrong, they cannot substitute professional lenses. But they can do a lot.
Lens performance is looked at from a very wide range of criteria. Some can be measured, others are more a matter of taste. The key ones are (not necessarily in this order) - build, speed (or how wide the aperture opens), distortion, centre sharpness, peripheral sharpness, sharpness at different apertures, colour rendition, ability to handle flare, chromatic aberration, bokeh, image stabilisation, zoom range, filter size, smoothness of movement, focus shift, focus breathing, and many others. Also, if you have a fetish for gadget looks then add finish and feel as well to my list above. And last but not the least, price.
Here I shall limit myself to my experiences with the key criteria.
Price is the single most obvious, and usually the starting point for most - whether amateur or professional. Kit lenses are a fraction of the cost of their equivalent professional brethren. So it's a no-brainer for the masses. But there are obvious reasons why they are relatively cheap - they perform relatively poorer on most counts. How much poorer is acceptable for you is obviously subjective; but is also the crux of your decision . And not all kit lenses are made equal, so get to know about them before you invest. Generally, you get what you pay for.
Sharpness - use within an f-stop range of f/8 to f/11 (stretch - f/5.6 to f/13)
Image quality encompasses a wide range of parameters, the most important being sharpness (both centre and peripheral) in my books. Most consumer lenses are designed to be their sharpest in the f-stop range of f/8 to f/11, and quite usable as wide open as f/5.6 or stopped down to f/13. But in my experience, the largest opening (say f/3.5) degrades image quality way too much. In short, the extra f-stop you gain is not worth the penalty paid in quality loss. It is true for both centre and peripheral sharpness; but the latter suffers most. However, it may or may not matter to your style of shooting. For example, if shooting portraits, often a little loss in periphery sharpness is of little consequence; but is critical for landscapes.
Similarly, openings smaller than f/13 lead to loss of sharpness all across due to a phenomenon of light called diffraction. But I very rarely find myself shooting at f-stops smaller than f/13. So I would not worry about this as much as I would for the largest aperture openings.
Build - kit (and consumer) lenses are less durable
Most often made of cheap plastic, they are more susceptible to knocks and breakage. So use them with great care. On the other hand, pro lenses are largely made of metal bodies; and barring physical abuse can last you a life-time!
Distortion - acceptable in most cases
In my books, distortion is of three types. Many softwares, and even cameras, allow you to correct or minimise these, but the 3rd is the most difficult to.
- The most common is wide-angle distortion and which is always the worst at the periphery. If shooting people, keep them away from the edges.
- Barrel or pin-cushion distortion.
- Complex or wave distortion. This manifests itself in the form of non-uniform waves, and the non-uniformity may even vary as you change the focal length. It is the most difficult to correct even with software.
Filter size - use a converter ring if needed
Good quality filters (polarising, neutral density, etc.) are expensive. So you would want to use the same filter (size) for as many of your lenses as you can. However, there is no standardization in lens diameter even with the same manufacturer. Hence, the best solution is to buy a larger filter (e.g. 77mm, a common size for many of the Nikon pro lenses), then use step-up rings to fit your smaller diameter lenses (e.g. the 52mm which many of the Nikon kit/consumer lenses have). Of course, this is only of relevance if you plan to use varied diameter lenses over time.
A common problem with kit lenses is that filters, which have metal threads, get easily stuck on kit lenses with plastic threads. This can break the plastic lens if the filter is not removed gently.
Other criteria ... varied, but mostly acceptable
The remaining criteria that I had mentioned in the earlier part of this article are most often within acceptable range in the newer kit lenses. And some of them (like optic stabilisation) are features that you may or may not want (varies with your type of shooting). But one has to do one's research before buying any lens, as each has unique characteristics and downsides.
Conclusion - not as junk as they are made out to be
Overall, when used within your lens' limitations, kit lenses serve well in many situations. While by no stretch of imagination do they match either pro or specialist lenses, they are often worth the low price you pay.
So don't ignore them ... understand them.
A side note
Kit lens are not to be confused with consumer lenses. While they overlap, kit lenses are generally those that are sold with a camera body. On the other hand, many more non-kit consumer lenses abound. One example of the latter is the Nikon 55-200mm f/4-5.6G IF-ED AF-S DX VR lens. This zoom lens is sold separately and is usually bought in addition to the kit lens (which most commonly covers the 18-55mm range).