Reflectors are useful for a lot of things, but there are some things that need to be kept in mind about them.
A photo reflector reflects light... but it doesn't reflect it like a mirror (unless you use a mirror or something highly reflective like that). When you reflect light from a reflector, you have a considerable amount of light loss. You can get light loss from the light being absorbed by the reflective material, loss of light from the light that goes through the reflector and loss of light depending on how reflective the material is. Then you have the loss of light that is reflected but is reflected at angles that does not hit your subject or object being photographed.
Color also needs to be taken into consideration. Of the 3 main colors, white, silver and gold, white is used the most because it gives (or should give) a neutral color reflection and won't put a color cast in the reflected light. Silver is a more reflective material and reflects more light, but it is also a harsher light and a cooler (as in color temp) light. It can cause bright highlights to form on your subject or object, which might not be what you want. Gold is used to produce warm tones, very much like the light you get from a sunset. The color cast this creates might not fit in with the color temps you are trying to create but is good for warming up an image.
Black reflectors are often used to absorb light and to block light.
Another aspect that people sometimes over look, is the size. A too small reflector is going to create a small source of light. In portraiture, it's said that your light source should be at least as big as your subject. The same goes for a reflector. Many people use reflectors in the 36" range. For people and larger objects, these are just to small to give a wider amount of reflected light unless you are trying to add light to a small area. If you have seen professional studios and outdoor setups, you have probably seen the 6' and larger reflectors.
Distance is where most seem to fall short though... If you have a 45" reflector and you stand 6' or more back from the subject, you are not getting a lot of light reflected on the subject. Most of it is being lost as mentioned above. Reflectors (in most cases) need to be used up close... right at the edge of where the picture is going to be cropped close. Yet many people not familiar with reflectors will hold them way back and then when they look at their images think that the reflector isn't doing any good. This is why a lot of reflectors sit in the corner unused... or are used but they really don't help the image.
Reflectors can be great for a lot of things... but like so much of photography equipment, you have to learn to use them.
Mike