Hi Mike,
I'm really sticking to a simple set of sent > opened > clicked (success) for ours. These are obviously based on how they interact with the emails. We are using lead scores to propel our prospects from one stream to another (we use General, Early, Mid, Late and Oppty Managemet) so I didn't want to get too granular with the progression statuses.
Basically, I'm going to look at the overall "Engagement Score" for the emails and content and only see an individual's engagement when I'm being alerted about them, i.e.: they're MQL or SQL.
Here's
an interesting discussion that was generally about Engagement Programs but evolved into discussing the progression steps - enjoy!