Since filters often improve the image, using several at once is always a good idea. Use high speed print film. Print film is much more forgiving of exposure errors, and fast film allows you to shoot in all kinds of light, improving your creativity. Always use a cheap polarizer. Don't waste money on a circular polarizer.
Many people have reported no difference, so why waste money.
All the polarizers I've seen are circular anyway, otherwise you couldn't screw them onto your lens. I don't know what these poeple are talking about. Popular photography have shown that you can still get decent results with two or three stacked teleconverters and they should know. Never trust autofocus. It is incapable of giving sharp images, as proven by Popular Photography in a recent issue.