Hope it's ok to post in such an old thread - you said this was aimed at helping people who might have been using Gimp for a while but don't use layer masks because they don't understand them. As someone completely new to Gimp (I installed it 2 days ago), I think you've saved me a LOT of future work.

I'm working through the continent tutorial by RobA and after reading that whole thread, all 39 pages of it, I came across this tutorial. It has helped me already - I have completed the new version of the mountain tutorial by RobA but want one of my mountains to have different colors. It was easy to do by just adding a layer mask over the mountains, duplicating the layer, removing the mountain in question from one layer mask and removing all but the mountain in question in the other layer mask. I was then free to experiment with noise, gradients, bumps etc on the one mountain without affecting the rest.

Considering I'm 2 days into using Gimp, I'm very pleased with being able to to this kind of stuff already - thanks to this and RobA's tutorial!