I took this shot yesterday with a polarizing filter on the camera. It hasn't been heavily processed (I added a very slight bit of contrast, but I left the saturation alone). As you can see, there's an even deeper sky than in some of Bryan's examples above. The great thing about CPLs is that you can vary the effect by how much you turn the dial. That said, given times of day and atmospheric conditions that aren't optimal, you'll never get quite the same effect.