One things I did notice is that the filters add a slight-to-medium magenta cast to the images. More noticeable when the filters are stacked, but still there even when using a low value filter. I guess the solution is to always use a 0.3 plain ND behind the graduated filter so that the whole image gets a magenta cast, then use a greycard to set the white balance to counteract it.
For these, though, it was enough to selectively apply a fix in Aperture to remove the colour cast, then a reasonable amount of dodging and burning to bring out foreground features that got caught up in the ND filter and overly darkened. Overall, I'm happy that I can make the best of these and get some more pop out of the skies in my landscape photographs.
Note that the colours are genuine, once the colour cast was removed. I'm not sure if it was the light, the filters or a combination of the two, but the images all have a sort of retro look to them that was not applied in post.
The full set, including the sunset picture I posted the other day, is available for your perusal.