I have the CP but don't have the comparison photo
just want to share the info with you.
CP is good to cut the reflection. So if you shot the photo where the area has a lot of reflection of it, it is a good filter. Since it cut the reflection, so it may make your photo underexpose depend on the density.
if you just the blue more on the sky, that you can do it simply with PP and no need to spend like 50 or 70$ for CP filter.
By the way, I am sure many of you will buy Hoya because the quality of this company is good. However, if you go on Ebay and search for Kenko, you will find the filter has the same quality as hoya for almost 40% off. Why? Because both Kenko and Hoya are the same company. Hoya is for oversea and kenko is used for local japan. There is no different expect the price.