The 50mm prime lens has certainly enjoyed a Renaissance over the past several years. Thirty years ago, the kit lens for an SLR was a 50mm f/1.8 lens. Over the past 10 years or so, this has changed in favor of a medium zoom lens (generally 18-55mm). Manufacturers seem to have decided this is a good choice for consumers, and we think it is! Bloggers and photo enthusiasts, however, have disagreed, and they've taken a stand by inundating the Internet with articles about the merits of what is now dubbed "Nifty Fifty".
First, what's good about it 50 millimeter lens? 50 millimeter lenses are generally sharper and faster than a typical zoom lens. By faster we mean that it will gather more light. A 50 millimeter lense is usually f/1.8 or faster which means it generally gathers up to two and a half more stops of light then a typical zoom lens. This equals 4 times more light (or more). Many say that if you have a Nifty Fifty you do not need a flash indoors. This is blatantly wrong. What it means is that you will be able to gather more light and the need for a flash goes down (a bit), but it does not eliminate the need for a flash or extra light altogether in most indoor conditions. Every lens has its limits and there will be plenty of dark environments where a flash is still necessary.
The downside of a 50 millimeter lens is it only does one thing. It only shoots 50 millimeters, unless you are able to get to exactly the right distance from your subject to compose your image... whereas a zoom lens has a variety of focal lengths available to the photographer. Therefore a 50 millimeter lens is an extraordinary burden on the photographer because it takes up more space in their bag than would a zoom lens. Further, because it is a fixed focal length lens you will be required to make more lens changes than if you were to use a zoom lens.
In conclusion, although a 50 millimeter lens has its merits like light gaining capabilities and sharpness, its main downside is that it is a "UNI-tasker"; it only does one thing.