Remember that the most significant results comes from pointing the camera at ninty degrees from the sun. Pointing the camera in the general direction of the sun, or directly away from it reduces the filter's effectiveness. Due to this effect, areas of a photo closest to 90 degrees from the sun will be affected more.
The following photo is a good example of what I'm talking about. This is Provia 100F film with a polarizer dialed in to its maximum. Notice that the upper right of the sky is, to my way of thinking, too dark. That corner of the sky was closest to the 90 degree angle and therefore was affected the most.
I usually shoot Provia 100F slide film and find that too much polarization will result in a purplish or nearly black sky, but I will sometimes dial in a bit to knock down shiny surfaces that would otherwise burn out or to darken the sky to a deeper shade of blue; sometimes this helps to delineate clouds a bit in a light blue sky. I'm sure that any of the super-saturated films, such as Fuji Velvia or Ecktachrome 100VS, would look pretty "neon" with polarization. I'd echo Gary's advice and try it to see what happens with your film.
Other places where polarization is handy in when taking photos over water. Reflections are reduced and water color deepened. If the water is very clear to begin with taking too many of the reflections off may make water simply disappear. That may be good or bad, but could result in a "fish out of water." See this photo for such an example:
I photographed this chum salmon in Alaska. The water was very clear and adding a polarizer took virtually all the reflections off the water. Looks suspended in the air doesn't it.
Gordon Dietzman