There are two basic kinds of polarizing filters. There are linear and circular varieties, and the choice between them depends on the kind of lens you are going to put it on.
The older, non-auto focusing, lens will use the linear polarizer. The circular polarizer was developed for the auto focusing lens. Using the linear on the auto focus lens will yield very undesirable results with the exposure.
I have recently discovered a new type of polarizer that is called Moose's Filter. It is a circular polarizer combined with a 81A warming filter which yields beautiful results.
A polarizer traditionally casts a bluish hue over the entire scene. This is very desirable for skies and water and landscapes, but the warming filter corrects the color temperature, adding a pleasing, warm color balance to the scene.
The greens and reds seem to "pop" with this filter also, so it adds a nice color saturation to the image. I have been finding though, that bracketing exposures is even more important, since adding this filter reduces the light passing through the lens, and throws off the exposure.
It stills works great with removing reflections as well. I have one for each of my lenses, and am very glad I decided to try one.