Circular polarisers are great for saturating colors such as greens and blues, creating contrast between clouds and blue skies and will also help eliminate reflections from water or glass. They will also block up to about 3 stops of light so can work as a lowish strength ND'ish filter.
Regards the dark corners, on a lens like the 10-20, if used at small aperture you can get vignetting - the solution is to shut down the aperture or use a thin mount filter - or both. This will help eliminate these problems.
I'm no PS expert, but i'm not aware of a quick computer fix that will achieve what a polarising filter will.