You know, really, using a CPL can improve color saturation in many situations...it can make the sky a little bluer, make the whiote clouds look better, make human skin tones look better, and it can remove dull, almost-invisible diffuse highights on many surfaces. But the thing is, removing the diffuse highlights on things can, sometimes, make the shapes of objects look just a little bit "weird".
This is difficult to describe in few words, but here goes. We often see things (animals, plants, cars) and see this diffuse highlight on parts of the surface, and those highligh cues give us clues as to the exact shape, or texture, of the thing. Eliminating TOO much diffuse highlight can make "some" things almost disappear, like glass windows, or the surface of a shallow creek...if we polarize away all or most of the highlighting, we lose the shape of some objects.
I'm just saying this: a CPL is not a panacea. it will not always give significantly better results than shooting with no polarizer, and at times, the improvement the CPL provides will not be worth the hassles. Framing up images with a CPL on is not as easy as with no CPL, since it's harder to see the image through the finder. I also think that for maximum impact, CPL filters work best when you bracket the effect, and take 3 or 4 frames for each shot. Polarizing away too much reflection can make shrubs and plants look, well, weird. There's a balance between polarizing away that last little bit of highlight reflection, and polarizing away just enough to actually improve the shot.