Check out these two shots and notice the difference in exposure of the trees and the foreground, especially the grass. The notice the difference in the details in the clouds. The clouds look better defined and more like "real life" than in the second shot.
Both pictures were taken on "M" for manual on a D200 with Nikon's 18-200 DX lens at 18mm. 1/200 second at ISO 100, F7.1.
On this first shot, I used the filter:
On this second shot, everything is the same, 1/200 second at ISO 100, F7.1, but with no filter.
While the sky is really very similar (only slightly over-exposed correctly for retaining details of clouds), the foreground is horrible.
I can't wait to start shooting this filter with some nice film!