I find a polarizer very handy when shooting a scene with wet foliage - the polarizer nearly eliminates the reflections from the leaves, which brings out the colors more. Also, try putting on the polarizer when shooting a rainbow, as you turn the polarizer, you will see the rainbow become much more vivid.
Since I use slide films that already have fairly high color saturation (Fuji Velvia and Fuji Provia) I don't very often use the polarizer in the normal blue sky scenes. You will have to experiment with the film that you use.
Also, remember that the polarizer will cause you to lose a couple of stops of light, which will cause your camera to go to a slower shutter speed.