As stated above, you should not use a polarizer when light is very limited. You should use a polarizer when the angle between you, the sun, and your subject is 90 degrees and you want to enhance the blues in the sky. You should use a polarizer when you want to reduce the glare from flat, reflective surfaces like water or glass. In other cases, it's a trade-off.
Even without obvious reflective surfaces, there is often glare present when you have sunlit subjects because little bits of the surfaces of objects in your picture might be reflecting light at you. A polarizer can help with that. In those gray areas between obvious polarizer need and obvious need for more light, it's a judgement call as to how much it will improve your image by polarizing the light, how much it will degrade your image by decreasing the light, and how much you want to hassle with it. Some people use them except where they can't; others use them only when they must.
One other caution is when using wide angles with lots of sky in the picture. The effectiveness of a polarizer falls off gradually as the angle described by the sun, the camera, and sky, gets further from 90 degrees. If the sky in your picture covers a wide set of angles, there can be a noticeable decrease in the polarizing effect across your picture. In my opinion, a sky that shifts in saturation often looks worse than one than one that wasn't well saturated at all.
I suppose that the obvious should also be pointed out. If you want reflections in your image, don't polarize them away. You might like the specular highlights coming off the water. You might want to show someone's reflection in a window. Just because you usually don't want something doesn't mean that you never want it.
Also, use a circular polarizer and not a linear polarizer. You can't tell by looking at them (linear ones are round and rotate too). A linear one will screw up the readings from your exposure meter and might even jack with your auto-focus.
