Agreed on the ND filters. So many uses. Regarding CPLs. I use mine ~25% of the time for my landscape photos. I use them to cut reflections off water, bring out the sky, or make colors pop a bit more.
An example:
Palouse Hills-7033 by kayaker72, on Flickr
BTW...I picked this shot not as I think it is so amazing that it will convince you, but it is an example of the effect. The sky towards the right begins to show what the sky really looked (lighter blue) like and towards the left of the photo the effect of the CPL (darker blue). CPLs tend to have an uneven effect on skies at wider angles, as shown here, but it still can be an effect I like.






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