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I have a CPL for my lenses. I don't use it often, but when I do I am always pleased with the effect. Not only does it deepen the color of the sky, it also removes tiny specular reflections from foliage, making green bushes and trees look more saturated. It does the same for flowers and just about everything but shiny metal. It also cuts the glare on water and lets you see through it.
If you want to get a good idea of what a polarizer does before you lay out the bucks for one (and don't get a cheap one), get a pair of polarized sunglasses and you can see the effect there. And while I don't recommend a cheap polarizer, cheap polarized sunglasses are OK for seeing the effect.
Also, if you do decide to buy a polarizer, buy a CIRCULAR polarizer. I don't mean a round one, I mean a Circular Polarizer (CPL) as opposed to a Linear Polarizer. Your SLR meters through a 1/2 silvered mirror, and a linear polarizer will screw up your metering.