a quick one before i leave house for the day..
what brand is the filter?
polarisers are good for reducing reflections from non-metallic
objects, e.g. water, windows, plastics, etc.. it's good in
situations like taking fishes, aquatic animals in pons.. or cutting
reflection from wet ground.. it can also make skies appear bluer,
and less hazier from the reflections of dust and vapour in the
air..
the common limitation is that it reduces your exposure by 1/2 to 1
stop depending specs of the polariser..
there are two type of polarisers. linear and circular.. if you use
auto-focusing and your camera meter a lot, buy the circular
polarised ones.. linear polarisers are used if you are not using
the camera meter or auto-focusing..
that's all i can think of now.. 
