The retired machines are a great way to learn obviously. But after completing one having to use the walk through write up or video walk through the first thing I want to do is practice what I have just learned on a very similar but non-identical box. Make sure that the lessons I learned are sticking and I am understanding how to use them.
It would be really cool if there was a button on retired machines that you have already pwned to see more retired machines that have a very similar attack path.