I bought a couple of filters off ebay last week, one is a polariser, the other is a skylight (1A). I took a couple of photos with and without the circular polarising filter and the results are below. I deliberately chose to shoot the sun in the image as I felt that this was going to emphasise the differences. The first image was without the filter, and the second was with the filter.


I think it has made enough of a difference for that filter to become a standard when I'm likely to be shooting the sky.